Previous Level Cost and This level cost in Item Cost table

Hi - In the cost table cst_item_costs there is one PL and one TL column for each cost element like Material cost (TL_MATERIAL, PL_MATERIAL), Resource cost (PL_RESOURCE, TL_RESOURCE) etc etc. I found that for BOM items cost gets populated in PL columns and for Purchase item cost gets populated in TL columns ? Could you please explain what is the significance behind the same.

Thanks

Deepak.