[kd5bjo]

> “functionality hinting” (I’m sure there’s an actual term for this, but I don’t know what it is)

“Affordance” is the term you’re looking for: https://en.wikipedia.org/wiki/Affordance#As_perceived_action...

[aGHz]

It seems the term is in fact "signifiers". From your link:

> Affordances determine what actions are possible. Signifiers communicate where the action should take place. We need both.