From the output of last on RHEL5
user3 pts/6 :pts/1:S.0 Sat Feb 26 04:04 - 05:40 (01:35)
user2 pts/7 :0.0 Fri Feb 25 21:32 - 04:53 (07:21)
user1 pts/6 some.domain.com Fri Feb 25 20:38 - 23:19 (02:41)
To my understanding, the 3rd column tells where the tty session was created from. It's likely that user1 logged in remotely from some.domain.com via SSH, and user2 opened a terminal window in X locally.
But what about user3? It looks like he created pts/6 from pts/1. How does that usually happen? And what does :S.0 mean?