A DKIM selector is a component of the DKIM (DomainKeys Identified Mail) email authentication system. It is a tag that is added to the DKIM record in a domain's DNS (Domain Name System) zone.
The selector is used to identify which specific public key is associated with a particular email message. This allows multiple public keys to be associated with a single domain, each with a different selector, so that the domain owner can change the public key without affecting ongoing email communication.
For example, if a domain owner wants to change the public key used for DKIM, they can create a new selector and associate it with a new public key in the domain's DKIM record. When the new key is published, the domain owner can begin using it to sign outgoing emails, and the selector can be used to identify which key is being used.
In general, the selector is used to provide flexibility and versatility in the use of DKIM for email authentication. By allowing multiple selectors, a domain owner can use different public keys for different purposes, or can update their public keys as needed without disrupting their email communication.