For crafting kits with multiple possibilities, only one crafting kit is required to perform 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 that could be camo paint or fake bushes for a helmet.
Tool kits are a collection of tools, these would be used to reduce the time it takes to craft items and crafting kits.