Using mask options
When you have an existing selection or mask in an image, that information is retained when the Liquify dialog box opens. You can choose one of the following mask options:
Replace selection 
Shows the selection, mask, or transparency in the original image.
Add to selection 
Shows the mask in the original image, so you can then add to the selection using the Freeze tool. Adds selected pixels in channel to the current freeze.
Subtract from selection 
Subtracts pixels in channel from current freeze.
Intersect with selection 
Uses only pixels that are selected and currently frozen.
Invert selection 
Uses selected pixels to invert current freeze.