A proprietary file format is a file format of a company, organization, or individual that contains data that is ordered and stored according to a particular encoding-scheme, such that the decoding and interpretation of this stored data is easily accomplished only with particular software or hardware that the company itself has developed. In contrast, an open or free format is a file format that is published and free to be used by everybody.
Some proprietary format may be documented by the developer and released with a note that the format is subject to change without notice, and that the file should only be read or written with libraries provided by the developer. In other cases, the specification of the data encoding format may not be publicly documented at all; in some cases, the format may only be released to those who have signed non-disclosure agreements. A proprietary format can also be a file format whose encoding is in fact published but is restricted through licenses such that only the company itself or licensees may use it.