A proprietary format is a file format which is covered by a patent or copyright which is intended to give the license holder exclusive control of the technology to the (current or future) exclusion of others . Typically such restrictions attempt to prevent Reverse engineering, though reverse engineering of file formats for the purposes of interoperability is generally believed to be legal by those who practice it. Legal positions differ according to each country's laws related to, among other things, software patents.
The opposite of a proprietary format is an open format.
One of the contentious issues surrounding the use of proprietary formats is that of ownership. If the information is stored in a way which the user's software provider tries to keep secret, the user may own the information, but have no way to retrieve it except by using a particular brand of software. Without a standard file format or reverse engineered converters users cannot share data with people using competing software. The fact that the user depends on a particular brand of software to retrieve the information stored in a proprietary format files increases barriers of entry for competing software and may contribute to vendor lock-in concept.
The issue of risk comes about because proprietary formats are less likely to be publicly documented and therefore less future proof . If the software firm owning right to that format stops making software which can read it then those who had used the format in the past may lose all information in those files. This is particularly common with formats that were not widely adopted. However, even ubiquitous formats such as Microsoft Word can not be fully reverse-engineered.