SEQ Files On Commodore machines, general data files have been given the SEQ extension. This of course differentiates them from PRG (program), REL (relative), and USR (user defined) files. This is where text is stored. SEQ files are also used for storing program related information. This is where the program can retrieve configuration data which has been saved to this file. How SEQ Files Are Stored On Disk When a sequential file is saved to a disk, the Disk Operating System writes specific information to the disk identifying the file, file type, contents, and where this information is stored.