On a coil zipper, the coil can lay on the inside or the outside of the zipper tape. This further categorizes this zipper type into coil-in and coil-out.

Coil-in zippers have the coil on the inside of the zipper tape so it's not visible from the outside when worn (sometimes referred to as "reverse coil").

Coil-out zippers have the coil on the outside of the zipper tape so it's visible from the outside when worn.

Coil-in and coil-out sliders have different slot orientation to accommodate the side of the tape on which the coil sits.