Separate Approval Limit For Different PO Category

Dear Team,

Earlier our client use to follow same hierarchy (position based) with different approval path for procuring raw material and service order.Even if there is slight change in position, Approval limit would be same for that position irrespective of PO related to Raw/Packing Material or Service Order. 

Now there is change in Approval Limit for different PO Category.Let say if the Level 2 position is having approval limit upto 5 lacs for approving PO of RM/PM Category.Same Level 2 position is now having authority to approve upto 2 lacs for Service PO.

Please guide us on how we can achieve this requirement because i checked only one document type for Standard Purchase Order can be created in Document Types section and if we assign approval group to that position that approval limit will be common for all the Purchase Order Category.