Question: How/what triggers the 'Order Item Intermediate Approval' notification to be sent?
Answer: Order Item Intermediate Approval is for approval plans that are multilevel. With a multilevel approval plan, there could be more than one approval needed. If this is the case, then all the approvals that are done excluding the last will trigger an 'Order Item Intermediate Approval' notification. The final approval will trigger the 'Order Item Approved' notification.
You can see after the first approval at level one an intermediate approval notification is created. Then after the second approval, the final approval notification is created.