What is

What is DKIM

DKIM signs a message. The public key is a TXT record under _domainkey. The signature's s= tag is the selector and d= is the signing domain.

foo.bar._domainkey.example.com

RFC 6376's own example: s=foo.bar and d=example.com are queried at foo.bar._domainkey.example.com. A valid signature is not, by itself, DMARC alignment.