The UnClip action cancels the most recent Clip action that has been generated by a Clip Brush. However, it retains any sculpting details added to the surface after the Clip was made.
It is very useful to add details or accurate strokes on a clean surface that was temporarily flattened by a Clip brush. Once these details have been added, using UnClip will restore the surface to its curved shape and move the new details to match.
The UnClip feature can be found with all other Clip brush options the Brush > Clip Brush Modifiers sub-palette. You can also access this sub-palette by pressing CTRL+Spacebar, which will pop up a floating menu.