An apparatus comprising: 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; wherein the selection circuitry comprises a plurality of nodes, each configured to select m output signals from n input signals provided to that node, wherein n > m; wherein the plurality of nodes are arranged in a tree structure comprising a plurality of layers including at least a first layer of nodes and a final layer comprising a final node, the first layer of nodes being configured to receive input signals indicative of items in the pool of items, and the final node being configured to output a signal indicative of the at least one selected item, the output signals from a given layer of nodes other than the final node being supplied as the input signals for a subsequent layer; and wherein 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, characterised in that: each node in the tree structure has the same values of n and m, wherein m=1; the pool of items comprises p items and the subset of the pool of items comprises s items; the gate node is positioned in a given layer which is y layers before the final layer, where when y=0 the given layer is the final layer itself and when y>0 the given layer is an earlier layer than the final layer; the gate node is responsive to the suppression signal having the first value to prevent x of its input signals from being selected, wherein x<n; and the position of the given layer is defined by x / n (y+1) = s / p. 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; wherein the selection circuitry comprises a plurality of nodes, each configured to select m output signals from n input signals provided to that node, wherein n > m; wherein the plurality of nodes are arranged in a tree structure comprising a plurality of layers including at least a first layer of nodes and a final layer comprising a final node, the first layer of nodes being configured to receive input signals indicative of items in the pool of items, and the final node being configured to output a signal indicative of the at least one selected item, the output signals from a given layer of nodes other than the final node being supplied as the input signals for a subsequent layer; and wherein 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, each node in the tree structure has the same values of n and m, wherein m=1; the pool of items comprises p items and the subset of the pool of items comprises s items; the gate node is positioned in a given layer which is y layers before the final layer, where when y=0 the given layer is the final layer itself and when y>0 the given layer is an earlier layer than the final layer; the gate node is responsive to the suppression signal having the first value to prevent x of its input signals from being selected, wherein x<n; and the position of the given layer is defined by x / n (y+1) = s / p.