Deep knockout cannot knock out to the Background layer when using a clipping mask
I have read the document. Here is what it stated:
Select Shallow to knock out to the first possible stopping point, such as the first layer after the layer group or the base layer of the clipping mask.
Select Deep to knock out to the Background layer. If there is no Background, Deep knocks out to transparency.
- Note:
When not using a layer group or clipping mask, both Shallow and Deep will create a knockout that reveals the background layer (or transparency, if the bottom layer isn't a Background layer).
Based on the document, I believe the normal behavior is to knock out to the Background layer when using a layer group or clipping mask. However, in my test, the deep knockout only works when using a layer group. It does not work with a clipping mask.
Reference:
https://helpx.adobe.com/photoshop/using/knockout-reveal-content-layers.html

