1. Purchase Order = Quantities to be received
2. Transfer Order = Quantities to be fulfilled
On Transfer Orders, it is the To Location affects the calculation of the On Order field.
When the Multi-Location feature is enabled on an account, the calculation of the On Order quantity is per Location.
The standard report to show On Order per Location is called Current Inventory Snapshot report and can be accessed by navigating to Reports > Inventory/Items > Current Inventory Snapshot.
The field Location On Order is also available when customizing an Item Saved Search.