Let's pretend you have many sets of decam full focal plane images which visit the same region of the sky. You would like to use BestSeeingWcsSelectImagesTask to select only 5 which have the best seeing, and make a coadd with them. So you configure nImagesMax=5, proceed to create warps and coadds... and get very sad when there are giant gaps of empty sky all over your coadds.
This appears to be happening because a decam dataId is an amalgamation of visit+ccdnum. So the image selector is naively choosing the 5 very best visit+ccdnum combinations to include in the warps, which will then be coadded.
This ticket is to fix the BestSeeingWcsSelectImagesTask (and perhaps the WcsSelectImagesTask too, if applicable) so it knows which dataIds actually overlap on the sky when making its selections.