A Specialised mental health service organisation may consist of one or more clusters of service units providing services in admitted patient, residential and ambulatory settings. For example, a Specialised mental health service organisation may consist of several hospitals (clusters of admitted patient service units) and/or ambulatory or residential service unit clusters (for example, a cluster of child and adolescent ambulatory service units, and a cluster of aged residential service units).

To allow service units to be individually identified, but still also to be identified as part of a hospital (for the admitted patient service setting), or as part of another type of cluster (e.g., other cluster types for ambulatory or residential service settings), a separate reporting level called Hospital for admitted patient service units and Service unit cluster for ambulatory service units and residential service units is necessary.

The concept of Service unit cluster only applies to ambulatory and residential service units. The equivalent entity for the grouping of admitted patient service units is the Hospital.

However, for some ambulatory service units, the service unit may 'belong' to a hospital that contains both admitted patient and ambulatory service units. In this instance, the service unit cluster identifier for the ambulatory service unit would be the 'hospital identifier'. Other groups of ambulatory and residential service units could also be usefully identified as clusters. For example, clusters may exist of groups of residential services for aged persons, or groups of ambulatory service units in particular geographical areas.

The complete identifier string, including State/Territory identifier, Region identifier, Organisation identifier and Service unit cluster identifier, should be a unique code for the cluster in that state/territory. Service unit reporting structures should be identical between all mental health collections (e.g., Mental Health National Minimum Data Sets and the Mental Health National Outcomes and Casemix Collection (NOCC)). If no cluster applies, set to 00000.