It's not always the case that your vendor can fill your entire order in one shipment, so the app is built to allow you to receive "partial" amounts of the items you've ordered.
As outlined in the Start Receiving an Order section of the "Receiving a Shipment from Vendor" chapter, to receive an item, simply enter the amount received in a particular shipment into the "Receiving" column. You want to enter the amount you're currently receiving, for example:
If you've placed an order for 5 of item A, but the first shipment you receive from the vendor only includes 2 of item A, you'd enter 2 into the Receiving column. When you receive a subsequent shipment with more of item A in it, you'd enter the amount of item A received in that next shipment, until you've received the full 5 that you'd ordered.
Once you're done entering received amounts for a shipment, click the "Receive" button. As noted in the example above, you can repeat this process for as many partial shipments that you receive for an order until all items have been fully received. Once you start receiving an order, and until it's been completely received, the order's status will be shown as Partially Received within the app.
If you're syncing orders to Retail, when you've received all items for an order, be sure to finish the order in Retail.