how do I conceptualize system & virtual users?

I guess this would be a common case, I am hoping for some final clarification.

a few Linux boxes share ldap (multi-master) backend that PAM/SSSD uses to authenticated users, and these LDAPs are also is used by Samba, users start @ uid 1000. Boxes are in the same both DNS and Samba domains.

Do I treat these users as system or virtual users from postfix/dovecot perspective? If it can be a matter of choice then which is better/best?

many thanks.