question about sys_context

I came across a question on another forum that left me scratching my head, thinking "I should know that but have never really looked at it."

Given the query

SQL> select sys_context ('USERENV','SERVICE_NAME') from dual;SYS_CONTEXT('USERENV','SERVICE_NAME')--------------------------------------------------------------------------------SYS$USERS

Exactly what is that telling me?  SYS$USERS is certainly not the service name of the database (at least not in the sense of a service name from the 'service_names' init parm.  A search of both google and MOS on 'SYS$USERS' turned up nothing useful.