Could you provide us with an example use case?

For instance, one case in which you don't know in advance what you need to subscribe to is a user portfolio, because new lines can enter or exit the portfolio at any time.
For such cases, we provide COMMAND mode: the client still subscribes to a known item, but this item represents the entire portfolio table, so that item updates may consist in adding or removing rows (obviously, the Data Adapter must forge the updates in the proper way).
You can check the case on the basic portfolio demo.

Anyway, none of these advanced features have been ported to objective C.