See
Link for past answers... but nothing definitive.
A distress call (situation where the aircraft requires immediate assistance) is prefixed: MAYDAY, MAYDAY, MAYDAY.
An urgency message (situation not requiring immediate assistance) is prefixed: PAN-PAN, PAN-PAN, PAN-PAN.
are the (ICAO?) definitions I work to. NB the "nature of the emergency" is
not the issue - but the
assistance you require.
More informally, if things are under control, I can adhere to ATC expectations, then a Pan is a heads up to leave me alone and let me sort things out, and I'll let you know when I can what I want from you (ATC). If I cannot comply with ATC expectations/clearance e.g. Emergency Descent, or do need
immediate assistance then a Mayday it is.
In our company a engine failure on a twin is typically a Pan, an unextinquished fire a Mayday. Fire goes out becomes Pan. Not getting assistance you require from Pan, upgrade
Despite some replies you will get, I would suggest it is not clear cut, and is as the situation appears to you, and evolves, on the day - but do NB the definitions above.
NoD