This attribute reports the total resident set size (RSS) of all the member processes of the respective zone. Thus, the value reported by this attribute is the sum of the private and shared RSS of all the member processes of the zone.
The private RSS includes the memory pages that are private to the respective project only. The shared RSS includes the memory pages that are shared among the processes of other projects as well. Thus, if a memory page is shared, it would be counted multiple times for different projects. When the total RSS for all the projects is added, it is possible that the resultant value is greater than either the total RSS of the parent zone, or the physical memory when projects from all the zones are considered. Hence, private RSS of zone is a better metric than total RSS.