The MendixSSOUsers and the Demo-users are two different things.
The MendixSSOUsers are the users that Mendix, your IdP (identity provider) in this case, has listed as users of your application. When those users log in they can choose to click the Mendix account button. If they are logged in via sprintr, then they will get authenticated right away, if not, they will get the sprintr login popup.
Demo users are always created by your application as ‘local users’, never as MendixSSO users. This makes sense. You don’t want to trouble Mendix with accounts that are only added to your application for demo purposes. That is why you have no entity ‘MendixSSO.MendixSSOUser’ in the dropdown.
So you are still left with the choice for your demo user Bianca, A.A or S.U? The System.User is the entity containing all attributes for the user-account (hoping that one day Mendix will see the light and rename ‘user’ to ‘useraccount’). A.A is a specialization of System.User, so it has inherited all attributes and added some of it of its own. The answer depends on what users this demo-user is for. Whatever choice is made for them, you should also make for Bianca. Simpel as that.
Ask your girlfriend. She should know it 😅