OPEN-22
Lists are open unless a rule says they are closed, and added names are registered.
What was open
Many rules listed the members of an object or the values of a field with the word "exactly", and nothing said whether an implementation could add to the list. Read as closed, the lists made every addition a breach, including additions that clients depend on and that break nothing. Read as open, nothing stopped two implementations giving one added name two meanings.
Decision
A list of members or values is open unless a rule says it is closed. An implementation may add members or values; an addition must not change the meaning of anything the specification defines, and a consumer ignores what it does not recognise. A rule that needs a closed list says "closed" and gives the reason a consumer depends on the list being complete; "exactly" no longer implies it. Added names are recorded in an informative register, extensions.md, without a required prefix: a prefix turns into a rename the day a name is standardised, as HTTP found with "X-" (RFC 6648). The specification never gives a registered name another meaning.