In the English translation, the Create Trap description sounds like the skill only calls items in the EXT-1 or EXT-2 slots. However, the Chinese instead just states it uses one of the EXT slots to create a trap (this is also how it actually works in-game). This is can lead to some very irritating misunderstandings about how the skill works.
Of course, in all honestly I would really, really prefer that Create Trap either only called EXT-1, or more desirably just straight-up only used Rocks, because it's a hassle managing the skill so I'm not, say, wasting a Frag Mine to do basic trap damage. Likewise, at least from my experience there's hardly ever any need to have ultra-heavy (or light) traps, so I don't really see the point in calling EXT slots for the skill to begin with.