It's not that black and white, Cosmo.
Consider a twin just took off, and the tower hears any of these three calls from the aircraft:
a. "Mayday. ABC123 one engine out, can't maintain altitude!"
b. "Pan Pan. ABC123 one engine out, can't maintain altitude!"
c. "ABC123 one engine out, can't maintain altitude!"
Which of the three must be treated as an emergency?
Under ICAO rules, all three.