Why DKIM needs a selector
A DKIM key is published as a TXT record at selector._domainkey.your-domain. The selector is a label chosen by whoever sends the mail, and a domain can have many: one per mail provider, rotated over time. DNS offers no way to list them, so a DKIM lookup needs the selector.
You find it in any message the domain has sent: open the message source and read the DKIM-Signature header. The s= tag is the selector and d= is the signing domain. If you leave the selector empty, OrbitProbe tries a fixed list of selectors that large providers use by default. A miss on that list does not mean the domain has no DKIM.