An Entry Sequenced Data Set (ESDS) is a type of data set used by the VSAM computer data storage system. 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

Sequential access

In computer science, sequential access means that a group of elements is accessed in a predetermined, ordered sequence. Sequential access is sometimes the only way of accessing the data, for example if it is on a tape...

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.