In addition to direct calls (that is, EX or CALLPGM) to
compiled procedures, compiled procedures can be created for use
in COMPUTEs, DEFINEs, and Dialogue Manager calculations. This allows
one to take what might be a complex task and simply it into a reusable
function. A classic example of this is a lookup function for strings
or statistical model that requires many inputs and yields a single
result.
The User Written Routines are also known as User Written Subroutines,
UWSR functions, FUSE functions, and FUSELIB functions in some documentation.
They are all the same feature.