How do we know what each variant does?

Hello MacPorts community, 

I have never used them, probably because I don't have an answer yet to that question.

What exactly are variants of a given port, and what are their description ? As an example, I don't understand
what the X11, the GUI, the NO-GUI variants are for the PSPP port, since the standard installation (no
variant) provides both non-GUI and GUI modes, the latter with X11.

Thanks for insights, 
Pat