It is fairly meaningless in a choreography description to state that:
A Participant Type identifies a set of Role Types that MUST be implemented by
the same entity or organization.  Its purpose is to group together the parts of
the observable behavior that MUST be implemented by the same process.

Both the restriction that they must be implemented by the same entity or
organisation and in the same process are implementation details outside the
scope of the specification (and neither need be the case).