Clipping can be used when you want to add some shadow, highlights, or a material pattern without it protruding from the undercoat layer.
Tap the ① Layer button to open the Layer window. Select the ② skin's undercoat layer, tap the ③ Add Layer button to add a new layer above the undercoat layer.
Tap on the ① the new layer to make it a Current Layer, then tap the ② Clipping button. When clipping is turned on, the layer will move slightly to the right. Finally, close the Layer window with the ③ ↓ button.
If we draw a line from the top of the undercoat layer, it will protrude from the undercoat if clipping is off, but if clipping is on it will not extend beyond the undercoat. I will use this to put shadows on my skin.