What collision is used for hit detection?

I am creating level meshes with UCX collision. I need to add physical materials for footstep sounds for this collision, but I see no way to do that. So I need to know what collision set is being used for footsteps-hit detection. Is it the simple UCX or complex collision?

How I’ve done it is triggering an animation notify when the foot hits the ground in the animation, check the physical material of the floor below with a line trace, and play the appropriate sound.