Security Assist uses session variables.
by default, the primary key value for the logged in user is stored.
To find the name of the session variable that is being created, go to the login page and double click the Security Assist Authenticate Uses server behavior. On the Third Page of the wizard you can see the columns that are being stored in session variables and the names being used. You can also select other columns here.
More information on this is available in the "User Level Authentication" tutorial on the Solution Recipe tab of the Security Assist support page: