An entry-sequenced data set (ESDS) is a type of data set used by IBM's VSAM computer data storage system.[1] Records are accessed based on their sequential order, that is, the order in which they were written to the file; which means that accessing a particular record involves searching all the records sequentially until it is located, or by using a relative physical address (Relative byte address, RBA), i.e. the number of bytes from the beginning of the file to start reading.[1]
Keys may be used to access records in an ESDS by defining an alternate index.[1]
Web site: MVS Forums - Technical FAQ's - VSAM. Mvsforums.com. 12 December 2017.