This menu contains commands for editing an existing mask. Molecules frequently crystallize in space groups where there are more than one molecule in the asymmetric unit. This can be used to advantage by averaging the electron density. One requirement for this is the creation of a volume containing the non-crystallographic asymmetric unit, called the mask. The user is encouraged to read the paper [9] to get an overview of averaging but also to read the original literature.
Once a mask file has been generated, it can be edited in O with some of the Mask menu commands. The commands available give complete control over the final shape of the mask.