They do not, they are simply flagged so both work as needed, the Editor has settings for what Units can do... Not all at one timehow does load and unload work at the same time ?

In this case, it means the Pack Mule can Load into a ship and because it can transport other units, the Pack Mule can Unload them as wanted.
timerover51... that's good, perhaps you can find some helpful things there.