This is a request to create a mask bit indicating pixels that should be ignored for all purposes, including object detection. The idea for this is images with large contiguous regions that contain no useful data. Specifically, a region with this mask bit set would be ignored in the object detection stages.
A specific use case motivating this request is the use of the LSST DM software on co-adds from sets of individual dithers (potentially ISRed and combined outside DM stack). These co-adds have effectively circular regions of useful validity, and corners that should be ignored. In principle, the variance of these regions should also be set high, but the proper estimation of variance in regions of little data is not always robust (and quite frankly is often done wrong by the user – including this one). The FITS image array must be a rectangle, so the use of a mask bit (which can be visualized as a plane in the mask) to indicate that these regions should be ignored at all stages would be the clearest way of marking this.