Difference between USING and CHANGING

Hi all,

What is the exact difference between USING and CHANGING, as in the below mentioned examples the functionality is same.

perform sub1 using p1.

perform sub2 changing p2.

form sub1 using x1.

endform.

form sub2 changing x2.

endform.

In both cases it is "By Reference". When we pass the values in any of the perform the result will be same. When both performs do the same job what is the difference between USING and CHANGING.

Please clarify my Doubt.

Thanks in Advance