UAR is cited in this book:
It's in Italian book, by the way; English translation is the following:
Even more curious is the practice of what is called Ultimate Aspect Ratio (UAR), which results from the merge of multiple sources with different aspect ratios - looking to get the largest portion of the image possible: a Blu-ray edition could be matted to 1.78: 1 and show a small amount of extra frame on both sides, while an edition more obsolete could have another matte (1.33:1) and therefore show conspicuous areas of the extra frame at the top and in the lower one. Therefore, the purpose of the UAR is to merge each frame of an edition with the same frame from another one.