IMHO, a display manager is not a login manager. The login manager handles just the part of the process that gives the user a place to login and interfaces with the OS authorization process to recognize the user. The display manager can provide a login manager, but primarily handles the starting of the rest of the graphical interface by starting a new session on a local or remote system.
In the first place a user is given the green light to use the system, but nothing further occurs. In the second, the user is given the interface with which to work.
I think we need to be a little more precise and clear, and there should be more details. - KitchM