[Mobhack]

The L,D sequence only works for troops of the same platoon - as mentioned in the G/G. On deployment they can be spread apart, in game the dismounts require to be in the same hex as the vehicles of course. The feature is also limited to full game owners, IIRC.

How else would the code be able to determine exactly what troops to go into the transports (or vice-versa)?.

It's mainly there as a quick way for you to bulk upload your mech platoons in the deployment phase. I use it all the time - select any element and hit L then D, unless I have a specific plan for who is to go in what vehicle, when I load individually. (there is an 'load all' feature - but it loads everything, and is really just what the AI does on deployment, made available for those humans who want a similar jumbled up mess.)

99.9% of mech/motor rifle platoon formations are infantry + transports. Any other mixes are for some reason known only to the OOB designer (e.g. too many elements to fit platoon limits?).