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I'm not sure what you mean. You can use both / either / neither as wish. Sure, you could load a mask from one into another, but then the values might not remap to what you'd like them to (obviously, since you made a mask for Purpose A and then using it for Purpose B).

Necessary? No. Its a good practice though to keep the files clean and so that remappings of pixel values don't cause strange behaviors when close to those static points in the map.