The new "max" properties for sessions, subscriptions, and monitored items were a great addition, but I think a common and sensible way for a server to limit MonitoredItems is per session, so that one client/session can't starve out other clients by hogging the entire global maximum.

All we have right now is MaxMonitoredItemsPerSubscription, which when combined with MaxSubscriptionsPerSession could imply a rather large number of items per session are allowed. IMO MaxMonitoredItemsPerSubscription is more likely to be used to indicate the max per subscription based on the technical implementation details of the subscription, and not necessarily to indicate that each session can have MaxMonitoredItemsPerSubscription * MaxSubscriptionsPerSession items.

In any case, adding this new optional property doesn't break anything in terms of backwards compatibility, and would be useful to further communicate the server limits.

It could be noted that if a MaxMonitoredItemsPerSession is defined, then MaxMonitoredItemsPerSubscription * MaxSubscriptionsPerSession should logically be equal or greater (if defined as well).