[jvanderbot]

The distinction is - if it runs over ssh (no x / graphics login) or on a headless machine - TUI

If it requires graphics login, even if it uses character layouts - GUI

IMHO the T/G is not for the display elements, it's for the type of session.

[tgv]

Not to put too fine a point on it, but X11 runs over ssh just fine. No "graphics login" required.