A file format defines the structures of the data in a file.
File formats can be:
- either "proprietary" or "public".
- either "closed specification" or "open specification".
A "proprietary" file format is a file format where "official" changes are restricted to one or more parties, often the intellectual property holder, e.g. the author. In contrast to a "public" (open or free) file format, where file format changes are open to the public, e.g. Request for Comments (RFC). This does not mean that all proposed changes to the format are necessarily adopted by the "official" standard.
A file format is considered "closed specification" when no "official" information about the format is published by the intellectual property holder. In contrast to "open specification" where the intellectual property holder has published the format specification or at least parts of it.