The phrase "proprietary format" refers to a specific way of organizing and storing information that is owned privately by a company or individual. It means the format is not openly available or widely used by others. Full definition
Companies are starting to realize that using your own proprietary formats for books and operating system are constantly changing. (goodereader.com)
Other ebook formats sold online use proprietary formats and are incompatible with most devices. (goodereader.com)
I know lots of folks refuse to pay for content in proprietary format, but I contend that $ 2 is a small enough investment that it's worth it. (thestranger.com)