How Do I Use The White Values Of A Black and White PNG To Restrict An Effect To That Area?

As the title says. I have here an image of something that I rendered in C4D, and an "Object Buffer" render of a part of my original image (it is a black and white png, I'll attach the images here). I am not sure about a proper workflow on how to restrict a specific effect to use the black and white png as a base to restrict its effect on the original image (white part is the only part that is affected, and the black pixels are untouched)

Thanks!