Hello,
- After unlocking and turning on "Body Markings" in the shop/unlocks popup : the girl's body will be covered with some on-body designs (can be seen better if taken off the cloths).
- Changing those designs to different ones (the tattoos ones, the onahole ones ..) : go to the cloth bundles (it's the 'hanger' icon, above save-load, on the left side of the screen), and select any bundle. Changing the bundle will also change the body markings
Cheers