Options - sign me in automatically confusion

Using v2009 Build 14.0.8064.206

I'm unsure of the difference, if any, between the logon screen sign-in
option "Sign me in automatically" (unticked in my case) and
Tools/Options/Sign In/"Allow automatic sign in when connected to the
Internet" (ticked in my case). I use 2 Live Id's.

As it is, an automatic signin does not occur when starting Messenger. The
last used ID does appear as a default, though.

If I tick the login screen "Sign me in automatically" then subsequent
restarts of Messenger uses the last ID AND logs in.

I don't understand what the tool option "Allow automatic sign in when
connected to the Internet" is intended to accomplish, unless maybe it's a
reconnect if connectivity is lost, maybe?

Can you explain the interplay between those options to me?