mm 0 means Unknown
The match method is not known or not reported.
When to send it
Send 0 when you pass an EID through from upstream and the upstream party did not say how it was matched, or when your own system cannot tell. It goes in EID.mm on the user.eids entry, and EID.matcher may be omitted with this value. Sending 0 is better than guessing, because a wrong method misleads the buyer more than a missing one.
Buyers treat 0 as no assurance and usually apply the most conservative handling they have for that ID source. It is confused with 1 (No Match), which is a positive statement that no matching took place, and with leaving mm out entirely, which buyers read the same way as 0.
Example
{
"user": {
"eids": {
"mm": 0
}
}
}Fields that carry it
Other values in id match methods
The id match methods list page explains the whole list and its common mistakes.