opaqueUserId

The opaque user id currently in effect for reported events, or null before setup has run.

This is not always the value passed to setup: UserIdentity.Unidentified falls back to the id already in effect, or to a minted placeholder when there is nothing to fall back on. Read this to reconcile reported events against your own records, and note that a placeholder will not audience-match - call setup again with UserIdentity.Identified once your own identifier is available.