What is the TAG ID at the end of an ads.txt line?
Part of Supply path transparency
The short answer
The TAG ID is the optional fourth field of an ads.txt line, called the certification authority ID. It identifies the advertising system named in field one within a certification authority, today the Trustworthy Accountability Group. It says nothing about the publisher's account, and IAB Tech Lab prefers that systems declare it in their own sellers.json file instead.
Key takeaways
- Field four of an ads.txt record is the certification authority ID, and the ads.txt 1.1 specification marks it optional.
- The ID maps to the advertising system in field one, so lines naming the same system should carry the same value when they carry one.
- The specification says ads.txt certification IDs are superseded by the identifiers object in sellers.json, and that the field may be deprecated.
- A missing TAG ID does not make a seller unauthorized; authorization rests on fields one to three.
What is the TAG ID in an ads.txt line?
Definition
- Certification authority ID (TAG ID)
- The optional fourth field of an ads.txt data record: an ID that uniquely identifies the advertising system in field one within a certification authority. The Trustworthy Accountability Group (TAG) is the authority the specification names.
The ads.txt 1.1 specification (opens in a new tab) defines a data record as up to four comma separated fields. Section 3.3 calls field four the Certification Authority ID and describes it as "(Optional) An ID that uniquely identifies the advertising system within a certification authority (this ID maps to the entity listed in field #1)." It names TAG as a current certification authority and says the TAG ID would be included there.
The same row adds two notes a buyer should keep in mind. Declarations of certification IDs "are superseded by the *identifiers* object in sellers.json", and the field "may be deprecated in a future version of ads.txt." Below the table the spec repeats the point: certification IDs in field four are connected to the advertising system, and it is preferred to declare them within the sellers.json file. app-ads.txt follows the same record format, since its specification says its structure and content are the same as ads.txt.
| Example line | Field four | What the spec's example shows |
|---|---|---|
| greenadexchange.com, 12345, DIRECT, d75815a79 | Present | A system registered with a certification authority, with its ID |
| silverssp.com, 9675, RESELLER, f496211 | Present | A reseller line can carry the system's ID too |
| blueadexchange.com, XF436, DIRECT | Absent | A valid three field line |
| silverssp.com, ABE679, RESELLER | Absent | The same system as line two, written without the ID |
Is the certification authority ID required?
No. The specification says there are three required fields, and that the optional certification authority ID is included in some of its examples. Its section 4.2 example is a single reseller line, redssp.com, 57013, RESELLER, with the note that the system "is not registered with a Certification Authority, so the optional fourth field is omitted." Google's tools follow the same rule.
| Source | What it says about field four |
|---|---|
| IAB Tech Lab ads.txt 1.1, section 3.3 | Optional; maps to field one; superseded by sellers.json identifiers; may be deprecated |
| Google Ad Manager, Validate ads.txt syntax | All non comment lines must contain exactly 3 or 4 comma separated fields |
| Google AdSense, ads.txt FAQs | Field four is optional: a unique identifier for the advertising system within a certification authority |
| Google Display & Video 360, Authorized Digital Sellers | Field four is an optional certification authority ID; its sample mixes four field and three field rows |
The Ad Manager syntax page (opens in a new tab) treats a line with two fields or five fields as an error, not a line with three. The AdSense ads.txt FAQs (opens in a new tab) list field four as optional. The Display & Video 360 help on authorized sellers (opens in a new tab) bases seller authorization on whether the seller ID is declared in the file, and its sample authorizes four exchanges, two of them without a certification ID.
How do buyers use the TAG ID?
Because the ID describes the system in field one, a buyer uses it to answer questions about the exchange or SSP, never about the publisher's account in field two. The account check is covered in the seller ID guide. The TAG ID check is a smaller one: is the system on this line the company it claims to be, and is it known to the certification authority?
Use a field four value in four steps
- Group the lines by field one. The spec says the ID maps to the system in field one, so the same system should show the same value on each line that carries one.
- Open that system's sellers.json. Look in the top level identifiers array for an entry named TAG-ID, the standard identifier name in the sellers.json specification (opens in a new tab).
- Compare the two values. If they differ, the spec's rule is that the sellers.json identifiers supersede the ads.txt field.
- Look the company up in the TAG Registry. Note its membership status and any certifications listed, with the date checked.
What does a missing TAG ID mean?
On its own, very little. The field is optional, the spec's examples include valid lines without it, and the spec prefers the ID in sellers.json anyway. The spec's own section 4.3 example writes one system, silverssp.com, with the ID on one line and without it on another. None of that changes whether the seller account in field two is authorized.
| What the buyer finds | What it supports | Next step |
|---|---|---|
| No field four, TAG-ID in the system's sellers.json | Nothing unusual; the spec prefers this placement | Use the sellers.json value |
| No field four anywhere | No conclusion about authorization or legitimacy | Look the company up in the TAG Registry by name |
| Field four matches sellers.json | The publisher copied the system's ID correctly | Record it and move on |
| Field four differs from sellers.json | An inconsistency, which the spec resolves in favor of sellers.json | Trust sellers.json, and tell the publisher's contact |
| Different values for one system across lines | An inconsistency within the publisher's file | Flag the lines; authorization still rests on fields one to three |
Where can buyers look up a TAG ID?
The TAG Registry (opens in a new tab) is the public lookup. TAG calls it the official database for the global TAG Community and says it lets visitors find partners, "including their TAG membership status and any TAG certifications they hold." A company must complete the Verified by TAG process before it can be listed.
TAG's Verified by TAG page (opens in a new tab) explains where the ID comes from: each verified company is listed in the registry "and given a TAG ID (a unique 16-character identifier)". That makes two separate facts for a buyer to record. A TAG ID shows the company completed TAG's identity check; any TAG certification it holds is listed apart in the registry and has to be read there.
What to record for each exchange on a plan
- The field one domain exactly as written in the publisher's ads.txt.
- The field four value, or a note that it was absent.
- The TAG-ID value from the system's sellers.json identifiers array, if declared.
- The company's TAG Registry entry: membership status and the certifications listed.
- The date each of the three sources was read.
What does a TAG ID review look like on a real plan?
Fictional example
A science center reviews field four on its Louisiana site list
An unnamed science center and planetarium in New Orleans promotes summer science workshops and evening shows on family, education and local news sites across New Orleans, Baton Rouge and Lafayette, in English. Its agency's checklist asks for the TAG ID of each exchange on the deal, so the review starts in the publishers' files.
On one family events site, ads.txt lists an exchange on three lines; two end with a field four value and one does not. The exchange's sellers.json declares a TAG-ID in its identifiers array that matches the two values. On a local news site, a reseller exchange's lines carry no field four at all, and its sellers.json declares no identifiers.
The planner records the first exchange's ID from sellers.json, looks the second company up in the TAG Registry by name and notes what it lists. Both sites stay on the plan, because the account IDs and relationship types check out on each line, and both exchanges' sellers.json files list the accounts.
How does PlacementGraph show who is authorized to sell a site?
Frequently asked questions
Why do all google.com lines in an ads.txt file end with the same code?
Because field four identifies the advertising system, not the publisher. Google's Display & Video 360 help shows the sample line google.com, pub-0000000000000000, DIRECT, f08c47fec0942fa0. The publisher ID in field two changes from site to site, while the certification value in field four stays with google.com.
Should a publisher add a TAG ID to each ads.txt line?
Only where the system has one and supplies it. The specification makes the field optional, ties it to the system in field one, and says it is preferred that certification IDs be declared in the system's sellers.json. A publisher that copies the value should copy it exactly as the system documents it.
Can field four hold an ID from an authority other than TAG?
The field is defined generally, as an ID within a certification authority, and the specification names TAG as a current one rather than the only one. In sellers.json the standard identifier names are TAG-ID and DUNS, the Dun & Bradstreet number, and systems may add custom identifiers that a consuming system might need.
Does a TAG ID in ads.txt mean the exchange is TAG certified?
Not by itself. TAG says each Verified by TAG company receives a TAG ID and a registry listing, while certifications are listed separately in the registry. A buyer who needs a certification should read the registry entry for it rather than infer it from the fourth field of an ads.txt line.
Sources
- ads.txt 1.1 specification, sections 3.3 and 4 (opens in a new tab)IAB Tech Lab
- sellers.json specification, identifiers object (opens in a new tab)IAB Tech Lab
- app-ads.txt specification (opens in a new tab)IAB Tech Lab
- TAG Registry (opens in a new tab)Trustworthy Accountability Group
- Verified by TAG (opens in a new tab)Trustworthy Accountability Group
- Validate ads.txt/app-ads.txt syntax (opens in a new tab)Google Ad Manager Help
- Ads.txt FAQs (opens in a new tab)Google AdSense Help
- Authorized Digital Sellers (opens in a new tab)Google Display & Video 360 Help
Researched and written by PlacementGraph Research. Read how we research, source and update these pages. Corrections: diana@placementgraph.com.