values of notify ":priority" tag

In draft-ietf-sieve-notify-01 the possible values of the ":priority" tag 
are "1", "2", and "3". There is a note about perhaps retaining "high", 
"normal", and "low" instead. I wonder why we're not using "urgent", 
"normal", and "non-urgent" as specified in RFC 1327 and re-used in RFC 
2076 and RFC 3261 (the latter also adds "emergency" but I don't think 
we'd need that for email notifications).
Peter

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