Conventions in depth
A jump to 4♣ over a 1NT opening that asks partner how many aces partner holds.
Gerber is used with slam-going values (about 18 or more) when the one doubt is the number of aces. It is needed because, with notrump agreed, 4NT is a natural, quantitative slam invitation and cannot also be Blackwood. (The same applies over a 2NT opening and over the 2NT rebid after a 2♣ opening: 4♣ asks for aces there too, and 4NT stays quantitative.)
If more is needed, a follow-up of 5♣ asks for kings on the same stepped scale: 5♦ = zero or four, 5♥ = one, 5♠ = two, 5NT = three.
With merely invitational strength, the app prefers the natural, quantitative 4NT; and when it already knows enough about the aces to place the slam itself, it bids the slam. So 4♣ is used only when the number of aces is the one thing left to find out.
Once a suit has been agreed, the ace-and-keycard tool is Roman Keycard Blackwood instead, and BidMaestro keeps the two apart automatically.
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