What is the "Customer" field for and how is it used when creating a Purchase Order in Quickbooks Online?

Good day, abid,

The Customer field on the purchase order is used when it is billable to a customer/project. If you want to associate a PO as billable to a customer, select the customer on that field.

Furthermore, if you copy an estimate to a purchase order, make sure to select the same customer in the Item details section.

To keep track of your purchase orders, you can run these reports:

1. Select Reports on the left pane.
2. Scroll-down to Expense and vendors.
3. Select any of the following reports to run or customize them:

• Open Purchase Order List
• Open Purchase Order Detail
• Purchases by Product/Service Detail
• Purchases by Vendor Detail

Check these articles to learn more about purchase order in QuickBooks Online: 

•  Create purchase orders in QuickBooks Online
• Apply a purchase order to a vendor transaction

If you have additional questions about purchase orders, don't hesitate to leave a reply. I'll gladly answer them for you.