Approval hierarchy means supplier can assign credit approval levels to individual manager users by a set $ value. 

For example: You might set your account up with three approvers based on different values;

Admin or manager user 1: Applications up to $5k - Minimum level of approval required

Admin or manager user 2: Applications up to $50k - User 1 and 2 to approve up to $50k

Admin or manager user 3: Anything up to $500K - User 1, User 2 and User 3 to approve up to $500k

Examples ; An application comes through for $40k - User 1 (first) and User 2 (second) have to approve. A $100k application all three Users are required to approve - one after the other, only being notified when it's your turn.