Connect to Server; authentication via AppleID?

When connecting to a server for the first time, you are asked how you would like to be authenticated: Registered user, AppleID, or Guest.

The registered user option is straightforward. You supply an account name and password known to the server. This might be a local user id on the server or a network user id if your network is running Open Directory.

The Guest option is straightforward as well. Either the server allows guests or it does not.

I can't find any documentation on the AppleID option. My best guess is that it uses the AppleID of the local account requesting access, and sees if a user account on the server is associated with that same AppleID. If there is a match, you get authenticated as that user. On subsequent connects, it seems to default to the AppleId method and no longer asks. (How do you get it to ask again?)

Can anyone explain how this REALLY works? I have noted that an AppleID is indeed associated with each user account (optionally) and you can check it in the "Advanced Options" in the Users and Groups panel.

[I'm using El Capitan and the server is my own OS X server.]