Function module

Hi All,

Could any one explain me the purpose of using the following additions in a fm :

1.' Using and changing'

Ex : form get_data using mat1 changing mat2.

2.'Changing'

Ex. form get_data changing material.

3.'Using' .

Ex: form get_data using material.

Please explain me in detail with good examples for each of the above.

I appeciate your help.

Regards,

Vishwanath.