Mainframes were originally supplied with limited main memory by today's standards and the amount of data to be sorted was frequently very large.
In 1968 the OS/360 Sort/Merge program provided five different "sequence distribution techniques" that could be used depending on the number and type of devices available.
[b] With the announcement of virtual storage operating systems, DOS/VS and OS/VS, IBM unbundled much of the software and offered chargeable sort/merge program products.
Indeed, most Sort/Merge applications use the wide range of additional processing capabilities, rather than purely sorting or merging records: the Sort/Merge product is a very fast way of performing input to and output from these functions.
[1] The program placed emphasis on sequence distribution techniques, which could be defaulted depending on the number and type of devices available, or could be specified by the user, for making best use of secondary storage "sort work" (SORTWK) files.