When a login ID is added to a server, SQL Server adds a row to master.dbo.syslogins, assigning a unique server user ID for the new user and filling in the login ID, the password, and the user's default language. The passwords in syslogins are encrypted.
The suid column in syslogins uniquely identifies the user on SQL Server. A user's suid remains the same no matter what database he or she is using. The suid "1" is always assigned to the system administrator. Other users' server user IDs are small integers assigned arbitrarily by SQL Server when the login ID is added.