Apple Discussions login issue

What is supposed to be inputted for the 'username' in the login field to access Discussions? Email address or the Apple ID alias?

I'd been using the alias as the login name for ages but after updating my Apple ID details (street address), Discussions no longer accepts the alias - for a few minutes, I thought somehow my Apple ID had become compromised until I finally tried the Email address and it took. ¿håå?

So...?