Difference between Pass-by-Reference and Pass-by-Value-Result:

1. Pass-by-value-result is more efficient in partitioned memory where a Pass-by-Reference could refer to a distant segment or page, causing a page fault.  Similarly if memory is accessible through a network, it would necessitate a delay for network access.
   
   
2. Concurrent execution could give unpredictable results.  Using Pass-by-Reference could end up giving values over some limit (see ticket sellers race-condition example).  Using Pass-by-Value-Result could end up with an invalid value, but it would always be less than the limit.
   
   
3. Consider also the following example, where Pass-by-Value-Result differs from Pass-by-Reference:

   PROGRAM
      VAR i,j: INTEGER;
   
   
      PROCEDURE foo(x,y)
      BEGIN
         i:=y
      END;
   
   
   BEGIN
      i:= 2; j:= 3;
      foo( i,j)
   END
   

   In Pass-by-Value-Result, x is a synonym for i on the call, but the value of x is never changed, so the changed value of i is restored to the original value when returning from the function.

   In Pass-by-Reference the change to i is kept back in the main program.