sqlplus /sqlnet question

    I have ntserver with 2 instances say aa and bb. the tnsnames and listener files
are working correctly and I can connect to both via sqlnet alias but here is my question.

    when I am on local the machine I can access either instance via sqlnet alias or
at the dos prompt I can set oracle_sid=instance name access is fine works fine.
when using gui's I found out by accident that using 2:instance name as
the service works ,eliminating using my alias . ex system/manager_at_2:aa
does anyone know where the hell the 2 comes from

                Joe g.