mm 1 means No Match
No matching took place: the ID came straight from a third-party cookie or the device's resettable advertising ID.
When to send it
Use 1 when the ID was read directly, with no linking step in between: a device advertising ID such as IDFA or GAID passed in an EID, or an ID provider's own third-party cookie read in the provider's domain. EID.matcher may be omitted, because nobody did any matching. Set EID.mm to 1 on that entry.
Buyers read 1 as deterministic for that browser or device and nothing wider, so they do not extend it across devices. It is confused with 2 (Browser Cookie Sync), where the ID reached the seller through a sync, and with 4 (Observed), which covers first-party identifiers set by the publisher. Device IDs normally pair with UID.atype 2.
Example
{
"user": {
"eids": {
"mm": 1
}
}
}Fields that carry it
Other values in id match methods
The id match methods list page explains the whole list and its common mistakes.