The digital book format originally used by Sony Corporation. It is a proprietary format, but some reader software for general-purpose computers, particularly under Linux (for example, calibre's internal viewer), have the capability to read it. The LRX file extension represents a DRM encrypted eBook. More recently, Sony has converted its books from BBeB to EPUB and is now issuing new titles in EPUB.
RB is a container for holding Rocket eBook content. You can use DRM but books that came today not in use. The container typically contains an HTML file with images of the same and a pseudo-OPF (.info) file with all metadata or at least this is what it looks like after being converted.