The Sort/Merge utility is a mainframe program to sort records in a file into a specified order, merge pre-sorted files into a sorted file, or copy selected records. Internally, these utilities use one or more of the standard sorting algorithms, often with proprietary fine-tuned code.
It is very frequently used; often the most commonly used application program in a mainframe shop.
Modern sort/merge programs also can copy files, selecting or omitting certain records, summarize records, remove duplicates, reformat records, and produce reports.
Sort/merge is important enough that there are multiple companies each selling their own sort/merge package for IBM MVS mainframes. The major ones are:
(Some of these also sell versions for other platforms, such as Unix, Linux, or Windows.)
There is also a discussion of mainframe sorting in the BatchPipes article.