Description of feature:
RSM is useful with Pubsub to request big nodes.
Salut à Toi and Movim are doing extensive use of Pubsub, notably for blogging. Large nodes are common, and RSM it a precious tools to requests items while doing pagination.
IMO, behavior is defined in XEP-0059 (https://xmpp.org/extensions/xep-0059.html), beside others:
+ Limiting the Number of Items
+ Paging Forwards Through a Result Set
+ Paging Backwards Through a Result Set