I don’t think that the initial part of App. E is clear enough on the semantics of the extension points (what can the sender do, what can the recipient do), and I would suggest the following text to replace the first sentence:
Extensions to this specification have a variety of places to alter the protocol, including many fields and objects that can have additional values in a registry (Section 11) established by this specification. For interoperability and to preserve the security of the protocol, extensions should register new values with IANA by following the specified mechanism. While it may technically be possible to extend the protocol by adding elements to JSON objects that are not governed by an IANA registry, a recipient may ignore such values but is also allowed to reject them.
I don’t think that the initial part of App. E is clear enough on the semantics of the extension points (what can the sender do, what can the recipient do), and I would suggest the following text to replace the first sentence: