It is meant as a permission key to allow the user to login into the application. Its purpose is to work with the Restrictions set.

So, in your security project you would create a key like "MyLoginKey", and in the AppMain.vb (program.cs if the case) ShowLoginAndInitMainForm, you would set the LoginSecurityKey:

Then, you would apply this key to your users, set restrictions to them (Monday to Friday, work hours, for example), and deny this Permission to them outside this scope.

So, they would only be allowed to login inside those days/hours.