For Approval of Purchase Order Documents we can set Hierarchies. There are 2 types of Hierarchies which are mainly used:
1. Employee-Supervisor Hierarchy
2. Position Hierarchy
Here we will see the setting of Position Hierarchy and approval of Purchase Order Using the Same. This is an excellent document which Explains with Screenshots how to set up Position Heirarchies.
Oracle purchasing: how to setup position heirarchy 2010-12-09
Oracle purchasing: how to setup position heirarchy