how do I conceptualize system & virtual users?

Fri Jun 19 13:08:13 UTC 2015

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.