Do you mean after you have recorded a Palette? If you want to remove information from a recorded Palette, you would delete the information in the Attribute Editor cell. MASK would be used "live" when first recording the Palette to keep information from being recorded in the first place. Does that help?
If you're looking to make big changes, you could recall the palette to the channels, then apply the mask, then rerecord the palette (replace, don't merge) it should "fix" the palette to what you originally intended. I hope this makes sense...
