The CTOC chunk functions mainly as an index, allowing direct access to elements within a compound file. The CTOC chunk also contains information about the attributes of the entire file and of each media element within the file.
To provide the maximum flexibility for defining compound file formats, the CTOC chunk can be customized at several levels. The CTOC chunk contains fields whose length and usage is defined by other CTOC fields. This parameterization adds complexity, but it provides flexibility to file format designers and allows applications to correctly read data without necessarily knowing the specific file format definition.