You can add a RGBA texture to use each channel like a mask. Currently you have up to 4 textures and a total of 16 masks for your landscape

When a texture mask is added, you can remove the one not used. TerraPainter won’t allow removal for used mask

TerraPainter will expose for each splat layer a list of added mask. You can select which you require , the channel to use, and if you need to invert

Below an example from a demo scene included in the package. Each channel has a different texture, with different rules applied ( background is black just to give better evidence of the result )