How GTM Inboxes uses this term
GTM domain guidance treats DKIM selectors as part of DNS authentication hygiene before an inbox batch can be considered ready for launch review.
Where selectors fit
Selectors let a domain support multiple signing keys. That is useful for rotation and multiple sending systems, but every active selector needs a corresponding public key record.
Operational next step
When DKIM fails, compare the selector in a real signed message with the DNS record. Then rerun domain checks before relying on the domain in readiness reports.