2.8 Compound File Range Lock Sector
The range lock sector is the sector that covers file offsets 0x7FFFFF00-0x7FFFFFFF in the file, which are just before 2GB. These offsets are reserved for byte-range locking to support concurrency, transactions, and other compound file features. The range lock sector MUST be allocated in the FAT and marked with ENDOFCHAIN (0xFFFFFFFE), when the compound file grows beyond 2 GB. Because 512-byte compound files are limited to 2 GB in size, these files do not need a range lock sector allocated. If the compound file is greater than 2 GB and then shrinks to below 2 GB, the range lock sector SHOULD be marked as FREESECT (0xFFFFFFFF) in the FAT.