You can create a default integrated login user ID that allows connection via an integrated login to succeed 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 successfully maps 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 successfully connect to a database if the database contains a user ID named Guest. The permissions and privileges granted to the newly-connected user are determined by the privileges granted to the Guest user ID.