An External Subroutine has got access to the Caller's GDA

where in the subprogram does not have.

The subprogram is like blackboxes as they are not aware of

the caller's environemnt. They take only the parameters and

process it and return the values. e.g. One natural

subprogram can be called through middle tiers like Entirex

and Vitria from a different environment(Lets say a dot net

or java developed web interface).However a similar

interface can not call a natural external subroutine.

As far as calling is concerned you use callnat for

subprogram and perform for external subroutine.