Good afternoon.
I have always wondered why personnel/passengers on an aircraft are referred to as "Souls".

It goes back hundreds of years to maritime terminology.  Can't really tell you more than that.

"Souls on board" is not anywhere in any of ICAO or FAA terminology for pilots, air traffic controllers, etc.....not that I remember at least.  Everything is "persons on board", "number aboard", etc--no reference to the words "souls."

Aircraft (trains and ships) occasionally will transport corpses- usually in a casket or other container.

"Souls" refer to the LIVING people aboard and are mentioned that way so rescuers know the number of (hopefully) living people that need rescue.

Spent 20 years as an AF navigator.  First in flight emergency in the T-29 trainer, declaring an emergency landing, the pilot stated "# of Souls on Board".  Was rather sobering to hear that.

"Souls on board" is quick to say and easily heard/understood over the radio.  Radio communications in aircraft (particularly non-airline) can be noisy, although less so nowadays, and hard to understand.

It’s really sobering when you’re the meatbag saying it.

This is how I understood it as well. Could have fresh dead on board and need to know the number that were casualties. Not that big of a deal if you're in an RV8, but if you're flying a C130 out of a warzone, it could be completely different.

It also provides a way to differentiate normal people from any gingers who might be on board.   Or so I'm told.

50 odd years ago I stepped into my first USAF fire station.  Shortly there after and inflight emergency was announced.  Now when they blow the horn (horn for aircraft, bells for everything else) they announce the type of aircraft, emergency, SOB's and other pertinent info.

I looked at Tech Seargent Bridges and he just sort of chuckled and explained that it meant "Souls on Board".  Now, not that big of deal on a Phantom, unless of course the rear seater had already punched out, but if it were something like a C-141 it was significant.

For the next five years I heard that term almost every shift.  With aircraft from civilian sail planes to BUFF'S.  And everytime I made darned sure I paid attention to that information.  Heck, it was really more important that things like external fuel and weapons loads.