A luminosity mask is basically a black and white rendition of the photo and these masks are great for allowing you to target tonal ranges with specific effects or filters. I quickly show how to make a few quick adjustments in Develop and then jump over to effects to use the Luminosity Mask feature.

Using the new Levels and Window controls in the masking panel will allow you to target your adjustments to just where you want them using luminosity masks.

Dan Harlacher ON1 Photo RAW 2018 Intermediate / Advanced Layers Develop Effects Masking