@Faye37
Stelios, @Letti0 is right in what she says but, when you grant a pre-approval a box will appear asking if you wish to block your booking calendar from all other requests for the duration of the pre-approval. By checking that box the only user who will be able to follow through with a reservation in that timeframe will be the pre-approved guest.
So, a pre-approval will only give that specific guest an exclusive right to book if you make the conscious effort to check that restriction box. Don't assume that because you have granted a pre-approval that stops other guests from booking. As Letti say, fist one to whip out the credit card gets the reservation.
Cheers......Rob