Modify screen 521 with material description for transaction MB21

Hello

I have to add a material description for the material added in transaction MB21 in the screen 521,when adding matnr,quantity, uom,etc.When entering the matnr i have to display the material description for the corresponding matnr in that particular row,in a separate filed somewhere to the right.

I have worked quite a lot with screens but apparently not enough, because i do not understand how this subscreen with the resb items has been built.There is no table control or any internal table behind that whole table ,there is only one structure called resb.

Can anybody help?

Thank you