'Default Material Request Type' in Item Master

How to set Default Material Request Type in the item master

I have 3 levels of bom
1.abc-1 (first level bom)
this item to be manufactured and stored
2.abc-2 (second level bom)
this item to be maufactured at the time of production
3.abc (sellable item bom) .

what should be the Default Material Request Type of these 3 BOMs and purchase raw materials in the item master