From the manual ...
syslogins contains one row for each valid SQL Server user account. On the SQL Server distribution tape, syslogins contains an entry in which the name is "sa", the suid is 1, and the password is null. It also contains an entry named "probe" with an unpublished password. The login "probe" and the user "probe" exist for the Two Phase Commit Probe Process, which uses a challenge and response mechanism to access SQL Server.
Greg.