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XCom is a more or less realistic tactical game. Realistic planet, environment, weapons and situations. As realistic as the capabilities of the contemporary computers and style of game allow. Heck, if you replace the aliens with terrorists, you can call it Rainbow Six tactical.
The realism in the game even hurt the gameplay experience. Like soldier inventory management, buying mags and reloading the weapons manually, come on. XCOM'12 streamlined all the excess "realism" away.
Anyhow Type isn't all that hard to implement. If you want to buy a Craft Type 2, the can can interate through the facilities and check:
Can fit craft? Yes/No
Can fit Type 2? Yes/No
Does have enough free capacity? Yes/No
The game doesn't really care which craft is stored in which facility. Just like it doesn't care in which General Store you hold what item. You have free space for the craft at the time of purchase, you are good.
I know the Craft nr can be modified and I did that. I could fit 8 cars and that was cool. I could also fit 8 space dreadnoughts, which was a bit weird. That was the point I realized, a Craft size would be a good addition.