The "spared" build was added in revision 1.5 to address the particularities of rewriteable media. This build adds an extra "Sparing Table" in order to manage the defects that will eventually occur on parts of the disc that have been rewritten too many times. This table keeps track of worn-out sectors and remaps them to working ones. UDF defect management does not apply to systems that already implement another form of defect management, such as Mount Rainier (MRW) for optical discs, or a disk controller for a hard drive.