This video provides an overview of the New Photoshop-style Layer & Clipping masks in 3DCoat and 3DCoat-Textura 2024.
They are also compatible with Layer Masks in Photoshop.
Clipping mask
You can access to Clipping mask on presing RMB over a Layer to show the Drop Down Menu: Navigate to Clipping Mask to assign a new clipping mask layer below the selected Layer.

A clipping mask layer makes an invisible cut-out area based on the layer below it for you to paint in.

Why would you need to use a clipping mask instead of a layer mask?
While they share some conceptual similarities, their applications are different. 
Layer masks use black and white to show and hide different parts of the layer, whereas clipping masks rely on the shape of the bottom layer to define the visible area of the connected layers.
As example on the clipping mask layer, select Multiply blending mode. Change the opacity of this layer to make your painted layer look shaded.
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