Function of the Texture tool Edit
Once a texture is added, it covers the surface of the whole layer. Nevertheless it will not yet be visible, because the density of the texture is zero everywhere. To make the texture visible, you change its density at the desired location. You don't paint the texture onto the surface, but the texture's new density.
(Therefore it is not possible to 'erase' a texture as you would do in a graphics editor. Instead, you change its density back to zero.)
If there are several textures to a level, there is an order from 'top' to 'bottom'. Any texture at the top with a high density will obscure textures at the bottom. You can change the order of the textures as required.
Loading a new texture Edit
Changing the order of textures Edit
A list of available textures Edit