The word “scumble” is an art term that has a lot of meanings. Sometimes, it means thin glaze with a mixture that contains white. Sometimes it means painting with a dry brush using a broken color effect. Sometimes it means other things. In fact, it means so many things that it really doesn’t mean anything at all unless you define what you are trying to say each time you use it.
Let’s just get rid of it. It causes more confusion than it’s worth. It should be replaced with terms like “glaze,” “broken color,” “drybrush,” and “velatura,” which actually do have coherent meanings.
Just say no.