Gah! So much confusing cross information going on! I'll have to make a debug test save and test it out myself, as who knows who is talking about which version. . .
I'll get back to you all on this, yeah
It's not that confusing if you remember the techno-babble Steve used when he set up the distinction. EDIT: This was a long time ago, but it should be unchanged. It's possible but unlikely that Steve broke it when he did the engine redesign a couple of years ago.
1) There's two types of engines: Military and Civilian. If a ship mounts military engines then it is a military ship for maintenance purposes. If a ship mounts civilian engines then it might still be a military ship for maintenance purposes, since it might mount a different military system such as a plasma cannon. As Jorgen_CAB pointed out it's the engine type, not the maintenance type that matters.
2) There's also two types of jump drives: Military and Civilian. The difference is that the wormhole created by a military jump drive is "more stable" than the one created by a civilian jump drive. (That's part of the technobabble.)
3) Military engines are "higher energy" than civilian engines, and so are more de-stabilizing to the wormhole they pass through. (That's the other part of the techno babble.)
4) Military jump drives produce a wormhole that is stable enough to handle a military engine; civilian jump drives do not.
So a military jump drive can be used for ships that have any sort of engine, while a civilian jump drive can only be used for ships with civilian engines.
That being said, there are two subtleties for which I don't remember which way it goes:
A) Tugs. If a tug with a civilian engine tries to tow a ship with a military engine through a wormhole, does it work? My strong but vague recollection is that it does not, even though one could argue that the military engine of the tow is turned off. One could technobabble that the properties of the drive that destabilize the wormhole are present whether the engine is operating if one wanted technobabble to justify this.
B) "Standard" jump vs. "Squadron" jump. The wormhole used in a standard jump is a lot more stable (long term) than the one used in a squadron jump, so there's a small chance that Steve set it up to allow a commercial drive to jump military engines. I'm almost positive this is not the case, but I don't remember well enough to be certain.
One more thing: You need to be careful that the size of the ship mounting the jump drive is AT LEAST as big as the jump drive capacity. The capacity of the wormhole is the LESSER of the size of the ship and the capacity of the jump engine. So if you put a 50K ton jump drive on a 40K ton ship, the capacity of the wormhole would be 40K ton, NOT 50.
John