And that's all to your credit. I'll try to provide whatever information I have.
Krastorio uses four categories*: universal-equipment, vehicle-equipment, character-equipment, and vehicle-motor. Character-equipment is irrelevant here since it only contains items for player-only use.
- "universal-equipment" can safely be used for everything, wheter players or vehicles, no matter which.
- "vehicle-equipment" includes things that are restricted to vehicles, but can be used for any vehicle.
- "vehicle-motor" includes movespeed-based equipment which can be used on most vehicles, but not on wagons for instance.
*I also saw it using the "armor" category for one or two player-only items (for instance, the exoskeleton I mentioned), but, yeah. Nothing to do with vehicles.
Given that, I added "universal-equipment" and "vehicle-equipment" to all 5 categories (car, tank, locomotive, wagon and artillery wagon) but "vehicle-motor" only for car, tank and locomotives for the reason mentioned above.
As for armor, it's apparently to ensure compatibility if another mod adds something with this category rather than if Krastorio isn't installed. Though that doesn't change much to the end result I guess.
Also, unless it's outside the scope of that mod, do you perhaps have a way to also add these categories (without changing the grid size) to vehicles which already have a grid?