Thank you for doing these new relation types. I can see how they would be very helpful.

I also looked at the -09 diffs - thank you for making those changes. They do improve the document. 

I do have some nits and questions, but none rises to the level of an issue. Please do the right thing. 

In section 2 and in section 7, "it's" should be "its". 

In section 6.1, I see 

      In addition to the values defined here any value defined in
      [RFC8288] may be used.  However these uses SHOULD be documented in
      an RFC updating both [RFC5545] and [RFC8288]

I have two questions - why normative language at all for this? it's a requirement for what people should do, not what the protocol should do. 

and why SHOULD? It seems that if this is something that ought to happen, I don't understand why allowing the possibility that it won't happen makes sense.

In section 8.2, I see 

         linkparam      = ; the elements herein may appear in any order,
                          ; and the order is not significant.

                          (";" "VALUE" "=" ("REFERENCE" /
                                            "URI" /
                                            "TEXT"))
                          1*(";" linkrelparam)
                          (";" fmttypeparam)
                          (";" labelparam)
                          (";" langparam)
                          *(";" other-param)

I'm asking this out of ignorance - is it obvious what happens if one or more of these elements appears twice? I'm guessing that it's not obvious, because the order is not significant, so one can't know a priori what an implementation would do. If that's the case, you might want to say MUST NOT appear more than once, to avoid indeterminate behavior.  But if this would already be invalid, that's fine.