Textures can be layered and blended together to generate complex results using any bitmap, color or procedulars as layers.
To start layering textures, select a channel (Diffuse, Reflectance, etc) that has a color, bitmap or procedural assigned to it, and from the Channel Properties panel, click on the hamburger icon and select "Mix with a ...".
After clicking on the 'Mix with a..' option, you will see the two texture layers one on top of the other. Deleting and moving layers is exactly the same as in Layers.
Each layer (except the one that is at the bottom) has two blending modes. Multiply and Add. You can switch between them by clicking on the icon.
Multiply uses the * icon while Add the + icon.
When the blending mode is in Add mode, it is possible to adjust the layer percentage.