BRU is one of a number of backup formats utilised by Digital's RSX-11M system (the name comes from 'Backup and Restore Utility', the application designed to read and write BRU containers.) BRU came later in the operating system's history than other formats such as FLX and DMP and is therefore somewhat more complex. BRU was used in environments where tape was the likely target, but you may also come across BRU format disk dumps of tapes.
(Is this the same format as that produced by the Tolis Group's BRU product ? I don't know.)