On what basis we should define item planning code

Hi,

On what basis we should define Item planning code value.

Also, please advise on what basis I can define my item as MPS item, MRP item or DRP item.

Example : CHAIR is finish good. LEGS and CUSHION are component items (Purchased items).

So, what planning code do you suggest for this. and how I can classify item as MPS/MRP/DRP item.

Thanks