Existing Wording: it marks it as processed using the tag ?? and the meta operation (POST to [base]/[type]/[id]/_meta where [id] is MTA id for the source message)

Proposed Wording: it marks it as processed (e.g. by using a receiver-defined tag and the meta operation (POST to [base]/[type]/[id]/_meta where [id] is MTA id for the source message))

Comment:

using tag is just one way of doing this, using some Workflow oriented resource (yet to be defined) may be another.