Much like DSM Trims, a DSM mask restricts the DSM algorithm to search only a portion of the image, and in this case, outside of the masked areas of the photo(s). This allows you to focus your DSM in a specific area of your photo(s) and can help reduce processing time by reducing the search area, as well as reducing noise and unwanted regions (such as windows in a building).
See Texture and DSM Mask image file names on Photo Properties.
To use DSM Masks in processing the ‘Extents from’ setting in the DSM Options Dialog needs to be ‘DSM Masks’.
The Extents setting is only available for the Paired Photos method. However, masks (which are created at photo level) will be used during MVS if they are defined, but again, as with DSM Trims, in most cases it is not necessary to limit the extents for MVS processing.