10 codes were developed because they are short and easy to pick out over staticky air. I don’t know for sure, but I assume it was either military or police agencies originally. They are still used by police agencies and dispatched truck drivers. My experience is with AAA emergency road service in San Francisco and Los Angeles.
There aren’t universal 10 codes however, most agencies use slightly different versions. Police agencies sometimes use up to 10-299, but private agencies generally only go to 10-99 officially.
10-4 = OK
10-20 = location
Those two are always the same, but the rest can be slightly different. It’s been a few years, and I’m a little fuzzy now, but an example would be in San Francisco we had one code for “I’m here” - 10-97, but in Los Angeles that was broken down to:
10-94 = at the location, no member, no vehicle
10-95 = at the location, with vehicle, no member
10-96 = going to knock on the door of a residence
10-97 = with member and vehicle
In both places 10-98 meant the call was over, while in SF 10-99 means in tow, and it isn’t used in LA.
The “unofficial” one that everybody knows is 10-100 - “out of the unit to pee”, and then of course some bonehead will use 10-200, which is just TMI.
There is also a very specific grammar to 10 codes. I wrote a paper on it for a linguistics class in college. My professor thought it was hilarious. For example you can ‘split’ some codes:
“show me 97”
“what’s your 20”
“show me 99 to member’s residence”
but you’d never just say “4” the way you’d say OK in English, or “36” instead of “10-36” (what time is it).
Random anecdote about AAA - they also use a phonetic alphabet - Alpha, Bravo, Charlie, Delta… but they are an ultra conservative organization, so we were not permitted to use the military phonetics for “U” and “W” on the air because unions and whiskey are bad, and members might be offended.
I’ll shut up now. Show me 10-19 for 7. (shutting down and heading home, SF style)