What are the valid users that need to exist on Mac OS X

For example with CleanMyMac username option I can see users like "Kerberos FAST Account", com.apple.calendarserver and large number of others all with system privileges with id 260 or below except for Message Bus which is 500 MacPorts also has a user with id 503. Anything below 500 has a "system" in name. All "real" users that I have created are above 500.

so what would be the list for valid users that I should see in the space below 500 in a standard Yosemite installation with OS X Server on it...