Postion Vs Job based heirarchy

 Hi,

We have a requirement . Our client is in to Car lease and rental/ Fleet Business and Service. The procurement of cars is done by fleet department and for spare parts by service department.  Which hierarchy should we use ?

For spare parts Store keepers at each work shop create Purchase order and report to thier Work shop manager and they inturn report to one Service Manager for only PO approval? so people holds the same position ( Store keeper and Workshop manager) If we setup Position hierarchy does all store keeper report to one workshop manager?? ( is that person selected as per the alphabetical order) , which shouldn't be bcoz each store keeper reports to his Manager.