Activity Streams/Primer/totalItems in Collection

The Collection type has an optional property totalItems.

For some collections, the practical number of visible items is less than the total number of items in the collection.

Consumers of collection objects should take this number to be an upper bound on the number of items in the collection. Mastodon, at least, does not recalculate the total number of items in the collection for the authenticated reader.

Producers of collections should note that providing a totalItems property that is greater than the total number accessible to the current user is leaking private information about the collection ("there exist objects in this collection you cannot read"). Therefore, producers that cannot correctly count the items accessible to the user should not provide this property.