A default integrated login user ID can be created so that connecting via an integrated login will be successful even if no integrated login mapping exists for the user profile currently in use.
For example, if no integrated login mapping exists for the user profile name JSMITH, an integrated login connection attempt will normally fail when JSMITH is the user profile in use.
However, if you create a user ID named Guest in a database, an integrated login will successfully map to the Guest user ID if no integrated login mapping explicitly identifies the user profile JSMITH.
The default integrated login user permits anyone attempting an integrated login to connect to a database successfully if the database contains a user ID named Guest. The authorities granted to the Guest user ID determine the permissions and authorities granted to the newly-connected user.