Difference between Requirement class which is in SD and Production planning

Dear All,

We are working on MRP, Transfer of requirements which is a integration part of SD and PP module. I want to know the difference of REQUIREMENT CLASS which is there in SD area and Production->Production planning->Demand Management->Planned independent requirement->Planning strategy-> Define strategy. Do i have to maintain controls in both (SD and PP)? Why SAP has given these two options.. if any one can explain in detail, that is going to be a great help. Thanks you so much.

Regards,

Reddy