I looked in Staredit & Maplantis wiki and also ran a Forum search in UMS...

Can somebody instruct me on the basics of dropship inventory systems?

Dropship inventory system involves locations that only detect units on the ground, not in the air. So when you unload a unit it detects it, but when it's in the drop it doesn't. You can do this by making a location, selecting it's properties and unchecking all of the AIR flags. The location will only detect ground units.

After you do this, simply make a bring condition and action, then follow it up with a AI script saying enter nearest transport.

Step 1:
find a map which has a dropship selection system.
I happen to have one here, but it's a stacked defense, so it no longer works properly.

Step 2:
Comprehend the triggers.

Step 3:
Mimic the triggers into your own map.

Essentially what you'll have to do is use the "enter transport" ai. If the player unloads a unit (brings unit to ground only location), tell the unit to enter the transport, and set a death count or switch to say that the action has occurred, so you can use it in future triggers. Alternatively, you can just detect the unit unloading in the trigger, but using a death count makes it much easier to make the trigger.

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i always had a problem with this system when i used hyper triggers

basically you have to detect the drop then load it up before the trigger cycle can begin again, sometimes no matter how much i ran the script it always repeated the trigger

so i just made a death counter that tells me to move the detection location away once the unit has been detected, then when no ground units are left, it moves back

i just thought u might wanna use that information when making w.e u makin, u may not have the same problems i always seem to run into tho

Did you try adding the AI script action into the item effect trigger?

Well usually you set a death counter to 1 when you detect dropship unit 1, to 2 for unit 2 etc. and execute the reenter AI.
Then another trigger runs the spell and resets the dc.
When you detect a unit while the dc is still set, then just let the unit reenter without setting the dc.

Easy to understand, works flawlessly --> use it!

Here's a tutorial on setting location elevation, for a dropship system you just have a location that only detects ground units, or one that only detects air units, then you detect if they bring a unit to the location, if it's ground only, or you detect if they don't bring a unit to the location, if it's air only, you do your actions, then run the Enter Nearest Transport AI, to make the units go back in.

Like Nude said, you may want to set a death count, or a switch, when the unit is detected outside the dropship, that way it isn't redetected next trigger cycle before it manages to re enter the dropship, and then clear the switch/death count once it gets back inside.