When a character enters "Cautious movement" mode, that button displays a 'radar sweep' to indicate that Caution is activated.
I request something similar for "Cast Spell". At the moment, the only way to find out whether a character can cast a spell is to click "Cast Spell" then see how many spell slots he has left, then click "Abort" (where "spell slots" has various meaning as per character class). It would be better and more helpful to have the "Cast Spell" button display the character's ability to cast spells (e.g. number of spell slots).