Base color and emissive colors get combined.
Unless you subtract the area of emissive from base color like suggested, it will always be offset by the base color.
The clamp is just 0 to 1.
Its to insure that after any math you don’t have negative values, or values over 1.
Many ways to do it.
Depends a lot on your initial mask.
A normalized value you plug into the base color or the emissive is usually positive.
You can do a power by 10 to make sure all values above 0 are completely 1, a clamp to cut off all thats over 1, and then use it to subtract from the base color.
If you need to subtract the opposite of the mask you just 1- the result.