Purchase Order workflow ( Transaction : ME23N) determines the approval path for the buying organization . It first goes to the buyer, buyer's manager & thereafter based on the cost center & the associated amount, it goes for approval through the Cost Center organization .

i.e,

Buyer ( Requestor in the procurement deptt. of the buying organization)

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                                |
                                v
                      Procurement Manager
                                |
                                |
                                v
                      Cost Center org. approval ( based on amount & Sign.Authority level)
 

Regarding , Sales Order( Transaction: VA01); it is created in the Sales Org of the company  & basically governs the pricing structure of the order .The routing of an SO is

                 Sales Org. ( based on the cost center of the sold item)

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                                |
                                v
                       Distribution Channel 
                                |
                                |
                                v
                           Division
                                |
                                |
                                v
                           Reg. Sales office
                                |
                                |
                                v
                           Sales Group
                                |
                                |
                                v
                           Sales Person