How does StockTrim get the Stock on Order from Cin7?

By default, the stock on order comes from your sent purchase orders in Cin7 that have not been received. The stock on order for an item will be the sum of all these purchase orders for that item.

For location specific items, this is sync'd with the "Incoming" field in Cin7. You can also configure to also include Holding stock in this as well (under Settings).

To update the Stock on Order figure in StockTrim you need to sync your product data from Cin7 (not purchase orders).  Click the products tickbox if not auto selected as below and then the Load from Cin7 button.