Alice clues green to Cathy, touching slot 3 and slot 4.
What slot is focused?
Is this a Play Clue or a Save Clue?
What card note does Cathy write on the focused card?
Slot 4 is focused (i.e. the chop card).
Since green 2 is played on the stacks, the card on slot 4 must be either a green 3, a green 4, or a green 5.
If it is a green 3, then this would be a Chop-Focus Play Clue. That is legal.
If it is a green 4, then it wouldn't make any sense, because green 4 is neither playable nor critical. (All clues must either be Play Clues or Save Clues.)
If it is a green 5, then it wouldn't make any sense, because green 5 is not playable. Green 5 is critical, but it must be saved with a number 5 clue as a 5 Save.
Thus, it must be a Chop-Focus Play Clue on the green 3.
Cathy writes a card note of "green 3" on her slot 4 card.