FAQ Q177: What is the custom unavailability script?
Please see the last paragraph in FAQ Q137
which explains the usage of the custom pricing script. Custom unavailability script takes the same parameters as the pricing script but only returns information about possibility to make the reservation (including the error message if reservation is not possible). The price is calculated separately, using the pricing manager. Of course all other criteria for availability are also considered (including unavailability rules of pricing manager or pricing script), availability of the resource in given time, and other restrictions defined for the resource.