When a user is assigned a database alias, the user's login ID is mapped to an existing username in the database. This is done by adding a row to the sysalternates system table.
When a user tries to access a database, SQL Server checks for the user's server ID number, or suid, in sysusers. If the user's server ID is not found, SQL Server then checks sysalternates. If the user's suid is found there mapped to another user's suid, the first user is treated as the second user while using the database.