Prototypes

I am converting my first ILE program to "/free" format and have
questions concerning prototypes and calls to external programs. 

The following generic subroutine (CALPRG) is used to prepare for,
execute, and follow-up from, 'Calls' to other programs:
        (This subroutine (along with 'StuffP' and 'Unstuf') is 12-15
years old and used in hundreds (thousands?) of our legacy programs)

        C               BegSr   CalPrg
        C*  Call an external program 
        C*    Prepare for call
        C               ExSr    StuffP
        C*    Call program in variable 'Called'
        C               Call    Called
        C               Parm                    Pa
        C               Parm                    Prm1
        C               Parm                    Prm2
        C*    Do follow-up
        C               ExSr    Unstuf
        C               EndSr

The 3 parameters are defined and used as follows:
        Pa:     128 characters - gets converted to/from an array of
sixteen 8 character elements

                containing subroutine names to be run by the program
being called.
        Prm1:   256 characters - generally holds a record format
        Prm2:   256 characters - generally empty, may hold a record
format

In doing my conversion to '/free', I find that I must use 'CALLP' and am
required to define a prototype for the called program.

First, is it possible to define a prototype for a 'variable' ?
Second, if I can, why would I 'want' to, other than that I 'have' to ?

Thanks in advance for any clarification.

Tim Kredlo