Compound file elements can be compressed. The ulCompressTech field of a CTOC table entry contains a FOURCC compression technique identifier for the corresponding compound file element. If the field is zero, the compound file element is not compressed.
The definition of a specific compression technique may specify that either the entire compound file element is compressed, or that some specific subset, for example one or more RIFF chunks, is compressed.
The ulUncompressSize field contains the number of bytes that the compound file element will occupy in memory after decompression. If the compound file element is not compressed, this field contain the same value as the ulSize field, which identifies the file size of the compound file element.
Compression techniques may demand extra header fields or extra entry fields for decompression parameters. Compression technique identifiers, and any new entry fields corresponding to decompression technique parameters, must be unique. See Registering Multimedia Formats for registration information.