In a slam sequence, when the bidding reveals one potentially weak suit (eg the opponents have bid it or the partnership has bid the other three suits), a raise to five of the agreed major asks partner to bid on with a suitable holding in the weak suit.
Continuations are:
Pass: no first- or second- round control.
5NT: King.
6 agreed suit: Singleton.
Any other bid: Ace or void in the suit.
For example:
West wants to play in a slam unless there are two fast heart losers and bids 5♠ to ask.