mm 2 means Browser Cookie Sync
The ID was matched through a real-time browser cookie sync between two platforms.
When to send it
Send 2 when the seller learned the ID through a cookie sync, the redirect-based exchange of user IDs that OpenRTB describes in its appendix on cookie-based ID syncing. The buyer-side ID in the EID comes from the seller's match table rather than from a cookie the seller can read. EID.matcher may be omitted with this value. The field is EID.mm.
Buyers know sync-based IDs age out as cookies expire and match tables are pruned, so they may give them less weight than fresh first-party IDs. The value is confused with 1, which is a direct read with no sync, and with 4 (Observed), which uses first-party storage instead of third-party cookies. Browsers that block third-party cookies by default, such as Safari and Firefox, produce few sync-based IDs.
Example
{
"user": {
"eids": {
"mm": 2
}
}
}Fields that carry it
Other values in id match methods
The id match methods list page explains the whole list and its common mistakes.