An 8-bit unsigned integer that contains the compression unit shift, which is the number of bits by which to left-shift a 1 bit to arrive at the compression unit size. The compression unit size is the number of bytes in a compression unit, that is, the number of bytes to be compressed. This value is implementation-defined. NTFS uses a value of 16 calculated as (4 + ClusterShift) for the CompressionUnitShift by default. The ultimate size of data to be compressed depends on the cluster size set for the file system at initialization. NTFS defaults to a 4-kilobyte cluster size, resulting in a ClusterShift value of 12, but NTFS file systems can be initialized with a different cluster size, so the value may vary. The default compression unit size based on this calculation is 64 kilobytes.