Input and output procedures for SORT verb are simply
user defined procedures (coded normally with para or sections) which are used in conjuction with SORT verb.
When used with SORT, input procedure will be run before the records are released for sorting. and output procedure will run after the sorting.
Much useful when you want to fileter out or process the record BEFORE or AFTER the sort.
Not to mention, external sort used in JCL is much faster and efficient ways to SORT data.
Please let me know if you require more clarification.
