An apparatus comprises interface circuitry to receive requests and selection circuitry responsive to the interface circuitry receiving a given request to select, from a pool of items, at least one selected item to be associated with the given request. The selection circuitry comprises a plurality of nodes arranged in a tree structure, each node being configured to select m output signals from n input signals provided to that node, wherein n>m. The apparatus comprises control circuitry configured to output, in dependence on a type of the given request, a suppression signal, and the tree structure comprises a gate node configured to suppress, in response to the suppression signal having a first value, selection from input signals received from a given portion of the tree structure to prevent a subset of the pool of items from being selected for at least one type of request.