The other most common thing to do with a Mask Texture is to store different kinds of Texture information into the individual R, G, B, and Alpha channels.
A perfect example of this technique can be found by looking at the SM_Chair Static Mesh that comes with the Starter Content.
You can find SM_Chair and all the content that goes with it by first selecting the Starter Content folder in the Content Browser and then entering "chair" into the search box.
This will display all the content that is related to the chair. If you do not see the chair, it probably means that you did not include the Starter Content with your project.
To fix this, you will need to either create a new project or try the Migrating Assets tool to move the chair content from another project
into this one.
Opening up the chair Material, M_Chair, we can see a perfect example of Texture Mapping in action.
In this Material, the Mask Texture,T_Chair_M, is used to control every aspect of how the chair looks.
From what sections should be metal or non-metal to what color those sections should have, the Mask Texture helps to define all of this.
In the following image, you can see a breakdown of how each channel of the Mask Texture for SM_Chair works.
On the Left-Hand side is what the Mask looks like as a Texture.
The smaller images that you see on the Right-Hand side show each of the individual channels and what area those channels will affect on the chair Static Mesh.
Here is a breakdown of what type of information is stored inside each channel of the chair Mask texture.
Red Channel: Ambient Occlusion information. In the Material, this is used to help add some surface variation to the base color.
Green Channel: Metallic Mask. In the Material, this is used to define which parts are supposed to be metal. This is also used to help define what color the metal should be.
Blue Channel: Non-Metallic Mask. In the Material, this is used to define which parts are non-metal. This mask also helps to define the color of the non-metal parts.
Alpha Channel: Whole object Mask. This is not currently used by the Material.
When all put together, the chair looks like the following inside a UE4 level: