Two channels, neither direct. user.eids 1 arrive in the request body from client userId modules. Cookie sync 2 is harder: redirects write matched UIDs to a uids cookie on the PBS host via /setuid. Trap: uids lives on a host neither publisher nor bidder controls. Sync needs a prior visit, so pageview one is always identity-poor. See 4.
FIELD NOTES
- uids cookie hits 4KB cap at ~30 bidders; user.eids has no cap
- ITP on Safari/FF blocks 2, no cookie sync
- Identity gap vs client-side cuts PBS CPMs by ~20% on the first visit