Don’t think of loading/unloading transports as using the units’ movement point, think of it as part of the transport’s special ability to carry units. A transport may load units up to its capacity, so long as those units have not moved yet during the turn, and may unload any amount of cargo (none, one unit, or both) once per turn (so no split unloading). However, units that have been unloaded are not able to move for the rest of the turn, and you may not load or move a transport after unloading.
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