This is a tool for troubleshooting when using Photo Mechanic with other apps. Find it under Tools>Mask/Unmask. It is not to be confused with the Photoshop® mask tool.
Mask/Unmask will hide or show certain types of information from JPEG, TIFF, PSD, and TIFF-based RAW photos. Data that is masked is not deleted, and can be unmasked to retrieve the original info (except as noted below). Masking simply hides the data from most applications by changing the normal tags or resource IDs to something different. Unmasking restores the normal tags or resource IDs.
Masking data can be useful to diagnose certain workflow problems. For example, if you suspect that some other application is having problems with embedded XMP data within a photo, you can mask the XMP data for a photo (or group of photos) and see if this fixes the problem. If masking XMP data fixes the problem, you can choose to not embed XMP into photos in the IPTC/XMP preferences. If that didn't fix the problem, you can unmask the XMP and look for another cause.
Additionally, If you mask IPTC and/or XMP in a photo and then add new IPTC info, you won't be able to mask or unmask IPTC and/or XMP because both masked and unmasked data exist. The same is true for ICC profile in JPEGs.