Katsu is breaded deep-fried pork or chicken. If you use pork, any cut of boneless loin meat can be used. If you use chicken, breast is used.
When you buy pork, it’s usually cut for pork chops, and they are perfect for katsu. But if you use chicken, you need to slice breast, because pre-sliced chicken breast for cutlet is too thin for this purpose.
Here I used chicken breast to show how to slice, but either you use chicken or pork, method is the same, and both turn out juicy and tender.