It's known as subTenant and you can get it if you've exposed system level custom properties for the given phase of the EBS.
It is not a part of the payload, but you can press it down on the blueprint/deployment/vm in a property when you make your request. Use the tenant/sub tenant in the form. Notice there is a bug in the BG group name, thou, but you can easily look it up via the group ID.
var vRAHost = Server.findAllForType("vCAC:vCACHost")[0];
entity = vCACEntityManager.readModelEntitiesBySystemQuery(vRAHost.id, "ManagementModelEntities.svc", "HostReservations", "ProvisioningGroup/GroupID eq guid'" + groupID + "'");
for each (groupEntities in entity)
{
groupLabel = groupEntities.getProperty("HostReservationName");
return groupLabel;
}