The specific criteria around these codes are going to be driven by organizational rules and circumstances and isn't really something we can standardize.  If you only have 3 concepts, then you can either pick 'urgent' or 'asap' and would need to make that decision based on context or just pick one arbitrarily.  We don't think there's any further guidance that can be provided at the 'international', domain-independent level. In many cases, the timing information of the request (e.g. occurrencePeriod) may be as or more helpful than priority because it indicates when the event needs to occur.