Load All is simply allowing you to use the AI's load method - which usually is
not what a human player wants!. Useful for system testers to get a quick test battle started is all the use I ever make of it.
The L/D combo is for humans (with the full game) - in the deployment phase it loads all passengers of a platoon up no matter where they are currently on the map. I use that function in
every battle bar defence to get my APCs quickly loaded up with the requisite passengers. In regular battle phases it will load up all passengers that are currently
in the same hex as their transports, and of the same platoon etc. As I often keep the grunts in the exact same hex as the rides, again useful. (I usually keep mounted infantry so at least as and until they have reached a point they are going to "go firm" on. Until then, I usually get out and have a look around from time to time - staying in the same hex as the ride so I can pop back in if somebody objects to them dismounting!
).