Because sometimes you want to make a sim with heterochromia, but you also want that sim to have cat eyes. The package includes every feline eye color I've made and affects the left eye, made with Teanmoon's excellent heterochromia mesh (link). The eye is found under skin details and works with everything except the left cheek mole, but it can't be used with most rings due to the texturing space it uses (except the eyeball ring, which feels like some kind of faustian bargain situation). They're also fully resizable and will function exactly like a regular eye.