ITU-T Recommendation X.215 describes the "capability data exchange" functional unit, which is realized via the CAPABILITY-DATA SPDU described in ITU-T Recommendation X.225. I can read in the specification that its purpose is so that data transfer SPDUs may be sent outside of an activity, but why is it called CAPABILITY-DATA? Is there some lore behind its name that is obsolete?
As for the TYPED-DATA SPDU, I don't understand its purpose at all. The specification says that:
The typed data functional unit enables the SS-users to transfer data in a manner which is not subject to the control imposed by the availability of the data token.
But isn't the point of tokens to restrict dialogue? It seems like the TYPED-DATA SPDU completely contradicts one of the main purposes of the session layer. For that matter, why is it called "typed data"? Is it because it is meant to be real-time user data, like what somebody might type on a keyboard, or something like that?