CALL vs CALLP with EXTPGM

 >> Given that there is little or no difference, then is the only advantage
to CALLP the compiler check of the field characteristics.

Not the only advantage no.  There is also the CONST keyword which can save
you from having to define temporary fields just to pass them as parms.  For
example when calling QCMDEXC you don't have to create a 15,5 field - you
specify that requirement on the proto and then pass an integer or a literal
(or a calculation if you wish) and the compiler does the work.  Makes for a
"cleaner" more obvious program.

Other benefits include being able to use a meaningful name (the call is via
the proto name not the "real" one), being able to use literals simply as
parms, etc.

In addition if combined with a procedure interface (PI) in the called
program you can be certain that read-only fields are not changed by the
called program.

See the good old (and oft referenced) RPG Redbook "Who Knew You ......" for
a complete into to prototyping.

Jon Paris
Partner400