When a program job is run asynchronously the 'Successful Exit Code' box is disabled. Is there any reason for this?

Yes, in async mode the program is launched and "let it go,", in sync mode the program is launched and the scheduler waits for the program completiong then checks the exit code.

: When a program job is run asynchronously the 'Successful Exit Code' box is
: disabled. Is there any reason for this?