This is how I see it: following common sense logic (and international operating procedures), the most important information should be in the beginning. Who am I calling, where am I, who am I, further details of my station:
K9EZ de M/YO9IRF/P would read: Calling K9EZ, I am in England, my callsign is YO9IRF and I am portable.
The fact that i’m calling you is the most important (to get your attention), the fact that I’m in England is more important than my callsign, the fact that I am YO9IRF is more important than the fact that I’m portable. If I am operating from another entity, the callsign should start with that entity’s prefix, followed by my individual call. If I am portable, I can add that extra bit of information at the end, after the important identification details have already been transmitted.
/P = portable = not at my primary location
/M = mobile = in motion
Stationary car is /P, operating on holiday is /P, using another ham’s station is /P.