Use texture channels to pack multiple textures into one.
Arm recommends texture channel packing because packing helps to save texture memory. Packing saves memory because you can get three maps into one texture using this technique. This means that fewer texture samplers are required.
This texture packing technique is commonly used to pack roughness, or smoothness, and metallic into one texture. But it can be applied for any texture mask.
Use the green channel to store the more important mask. The green channel usually has more bits. This is because our eyes are more sensitive to green and less sensitive to blue.
The following image shows an example of texture packing:
Further reading on the color sensitive cones can be found here.