mm 3 means Authenticated
The match was based on user authentication, such as an email login or hashed personal data.
When to send it
Use 3 when the ID was resolved from something the user supplied by signing in: a hashed email address, a phone number, or a login on the publisher or a partner. Name the company that did the resolution in EID.matcher, since buyers judge the match by who made it. The value goes in EID.mm, often next to UID.atype 3 for a person-based ID.
Buyers usually value authenticated matches most because they are deterministic and can extend across devices. That premium is why 3 is overused. It is confused with 4 (Observed), which has no login behind it, and with 5 (Inference), where an ID graph links devices without authentication. Send 3 only when the request in hand was matched through an authenticated event.
Example
{
"user": {
"eids": {
"mm": 3
}
}
}Fields that carry it
Other values in id match methods
The id match methods list page explains the whole list and its common mistakes.