what all does instance include in user principal name - username/instance@REALM

[Priya Govindarajan]

Hi,

I have been using user principal name as  - usr1 at REALM , usr1/host at REALM, 
usr1/admin at REALM. 

In general, I understand that instance is the host name . What all does 
instance include here in user principal name ?. 

Any link to more information will be very helpful.

Thanks,
Priya