We will be providing guidance around the use of messaging vs. simply posting a Communication with payload.  Typically messaging is used for notifications that need to push numerous resources with a set of context.  Communication (when used with data as a payload for sharing as opposed to recording prior sharing) is generally used to respond to a request, though it can also be used to tie un-requested information to an existing context.  E.g. "Here's some supplemental information to use in performing a previously requested referral" or "Here's supplemental information to use in processing a claim".  We will update the specification to make the expected use clear.