“Exclusion” is normally defined as a jump to a non-trump suit above the keycard-ask. So, for example, if the auction goes 1–2–2–2–3–5, that would be the keycard ask with spades as trumps, excluding the ace of hearts (because responder is void in hearts). Whether or not you play 1430 or 3014, the responses to exclusion are typically 3014 (although obviously that would be a question of partnership agreement). In this case, if opener has zero keycards, the response would be 5so responder had better have at least one keycard himself
What about the situation where a void has been shown and then that hand asks for keycards? Let’s look at an example: 1–2NT*–3**–3–4***–4–4NT. In this sequence, 2NT* is Jacoby; 3** shows shortness in diamonds; 4*** shows a diamond void. Obviously, the 4NT bidder can’t want to know about the A, so you should answer with the appropriate “exclusion” response, even though, in this case, 4NT isn’t a jump.
This case was clear. But there is also the situation of an inferred void. Howard Piltch taught me this many years ago and it’s stuck with me. Here’s a real life situation:
You pick up: AQJ2 9432 KJ K94 and open 1. Partner responds 1 and you raise to 2. Partner now bids 3, showing a splinter in spades. Whoops! Most of the value of your hand just went out the window. In my opinion, you should now put the brakes on and bid 3NT (unless that would be treated as serious or non-serious 3NT). In any case, you decide to sign off in 4. But partner’s not done, much to your dismay. 4NT quoth he. Now, look at it from his point of view. He didn’t see the need to bid controls, rather he bid 3. That sounds like he’s asking your opinion: do you have wastage in spades. You surely do so you signed off! But partner doesn’t care. He’s hell-bent on bidding a slam provided we have three of the four significant keycards. Get that: he doesn’t care about spades–he has a void there. If he only had a singleton, would he first ask your opinion and then ignore it? No. He’d bid 4NT directly over 2–or maybe he’d make a control bid or something. So, you can safely infer that he has a spade void. No other bidding sequence makes sense.
So, now you tell him you have zero key cards (using the exclusion responses if you’ve agreed to play exclusion).
At the table, my partner didn’t pick up on the inference (I know it’s subtle) and told me about the A. I needed one keycard (the K) for the contract of 6. I still had chances though: I needed the king to be doubleton. It wasn’t, but my slam was still a 70% slam: 50% that he’d have the “right” key-card and, 20% in the zone. So I don’t feel too bad about it.