An apparatus comprising: processing circuitry to process instructions in one of at least three domains each associated with a corresponding physical address space, and to issue to a memory system a memory access request specifying a target address indicative of a target memory system location; partition identifier selection circuitry to select a partition identifier based on programmable partition identifier selection information associated with a current software execution environment which caused the memory access request to be issued; and partition identifier space selection circuitry to select a selected partition identifier space from among at least three partition identifier spaces based on a current domain of the processing circuitry; in which: the selected partition identifier space and the partition identifier together represent information for selecting, at a memory system component of the memory system, a set of one or more memory system resource control parameters for controlling allocation of resources for handling the memory access request or managing contention for said resources, or selecting, at the memory system component, whether performance monitoring data is updated in response to the memory access request; and the processing circuitry is configured to issue the memory access request to the memory system with the memory access request specifying the partition identifier and a partition identifier space indicator comprising a plurality of bits indicating the selected partition identifier space selected by the partition identifier space selection circuitry. processing circuitry to process instructions in one of at least three domains each associated with a corresponding physical address space, and to issue to a memory system a memory access request specifying a target address indicative of a target memory system location; partition identifier selection circuitry to select a partition identifier based on programmable partition identifier selection information associated with a current software execution environment which caused the memory access request to be issued; and partition identifier space selection circuitry to select a selected partition identifier space from among at least three partition identifier spaces based on a current domain of the processing circuitry; in which: the selected partition identifier space and the partition identifier together represent information for selecting, at a memory system component of the memory system, a set of one or more memory system resource control parameters for controlling allocation of resources for handling the memory access request or managing contention for said resources, or selecting, at the memory system component, whether performance monitoring data is updated in response to the memory access request; and the processing circuitry is configured to issue the memory access request to the memory system with the memory access request specifying the partition identifier and a partition identifier space indicator comprising a plurality of bits indicating the selected partition identifier space selected by the partition identifier space selection circuitry. the selected partition identifier space and the partition identifier together represent information for selecting, at a memory system component of the memory system, a set of one or more memory system resource control parameters for controlling allocation of resources for handling the memory access request or managing contention for said resources, or selecting, at the memory system component, whether performance monitoring data is updated in response to the memory access request; and the processing circuitry is configured to issue the memory access request to the memory system with the memory access request specifying the partition identifier and a partition identifier space indicator comprising a plurality of bits indicating the selected partition identifier space selected by the partition identifier space selection circuitry.