[Fyron]

The item in planetary cargo with the fewest hit point total is destroyed first, regardless of what type or size of unit it is.

But, IIRC, the Last patch made it so that weapon platforms are always destroyed first so that troops can survive bombardment and be useable for invasion defense. They are still destroyed in the same order, and other cargo is then destroyed in the same order when all WPs are gone if the attacker keeps bombarding the planet.