The _PERF_INSTANCE_HEADER structure is used at the beginning of an instance block that is returned when enumerating counterset instances or when returning performance counter data from multiple instances.

 typedef struct _PERF_INSTANCE_HEADER {
   unsigned long Size;
   unsigned long InstanceId;

Size: The total size, in bytes, of the structure and the instance name.

InstanceId: The counterset instance identifier. Each active instance of a counterset can be identified by the combination of its instance name and instance identifier. Two active instances of a counterset SHOULD NOT have the same combination of instance name and instance identifier. <3>