Editing Job Entry Subsystem 2/3 (section)

=== Customization ===
Source code was provided to IBM customers for both ASP and HASP, and many customers made substantial enhancements to these programs, some of which were incorporated into the official product.{{citation needed|date=May 2018}}  Far more installations made use of HASP than ASP, and in contemporary z/OS systems, there are many more JES2 installations than JES3.{{citation needed|date=May 2018}}  Because of their unique history, IBM continues to ship JES2 and JES3 [[source code]] instead of [[object code]], unlike most components of the operating system.{{citation needed|date=May 2018}}

To improve maintainability and serviceability of user-written enhancements, JES provides a set of exit points that pass control from the JES to user programs at key points of processing.{{citation needed|date=May 2018}}  These extensions can provide custom functionality such as special commands, custom print page headings, and non-standard job processing.