What is DomainKeys/DKIM Selector:

To support multiple concurrent public keys per sending domain, the DNS namespace is further subdivided with "selectors".
Selectors are arbitrary names below the "_domainkey." namespace. For example, selectors may indicate the names of your server locations (e.g., "mta1", "mta2", and "mta2"), the
signing date (e.g., "january2005", "february2005", etc.), or even the individual user.

The most important thing is: selector indicates your DomainKeys/DKIM public key location. For example: if your domain selector is: "s1024", your public key record
will be "s1024._domainkey.yourdomain"; if your domain selector is: "mta1", your public key record will be "mta1._domainkey.yourdomain".

Configuring DKIM with Spam Experts:

Log into your SpamExperts account and look under Outbound for Generate DKIM certificate.
Choose the key length and type in a selector name (like 'sel1', '3453', 'mta1', etc.).
Click on Generate and save new private/public pair.
In your DNS zone configuration, the Host value will be entered like:

<selector>._domainkey.<domain>
or
sel1._domainkey.domain.com

The Txt area value should contain the entire public key output, starting with any variables (like 'v=DKIM1').