For kits with multiple possibilites, only one kit is required to preform a crafting.
For Armorer, Plastics, either a Plastics Forming kit, or a Armorer, Plastics kit is required, never both. The utility of the kit is used for the utility of the crafting kit.
Kits are defined in Book 3, but the scope of kits is not completely defined. We are assuming that kits exist for any skill and that those kits can be used on the skills they are created for, even when that isn't exactly apparent. For example, a U3 Stealth Kit seems stupid, but however, that could be camo paint or fake bushes for a helmet.