A bidder that cannot recognize the user is bidding half-blind: it cannot apply audience data, so it bids low or not at all. Identity is the plumbing that lets buyers know who they are bidding on, and as cookies fade, it is where match rate, and CPM, is won or lost.
Key idea
The chain is match rate, then bid density, then CPM. A higher match rate means more bidders recognize the user, so more of them bid, so the auction is denser, and more competition lifts the price.
The User ID module
Prebid’s User ID module runs identity submodules that resolve and attach IDs. The common ones: UID2 / EUID (a deterministic, email-based ID), ID5 (a deterministic-plus-probabilistic graph), and RampID (LiveRamp’s people-based ID).
The resolved IDs pass into the bid stream as eids (extended IDs) on the OpenRTB request, so every bidder sees the same identity signals.
| Traffic | What carries identity |
|---|---|
| Logged-in publisher | Deterministic IDs like UID2 shine: real, email-based identity. |
| Anonymous traffic | Probabilistic IDs like ID5 do more of the work, since there is no login signal. |
Quick check
Why does a higher match rate raise CPMs, not just fill rate?
Note
Consent gates identity: if the user withholds consent, the IDs never populate, a real "the ID is missing" cause. The Identity analyzer reads which IDs a config carries.