EBS R12 - Accounts Payables; what is the field Shipped used for?

In the invoice workbench when we get in the "Match to Purchase Orders" form there is a section called "Shipment Quantity". In that section there is a field call "Shipped". I need to understand what that field is made for.

User guide says it is "The number of shipped purchase order lines".

1st question, What is it exactly?

2nd question, What kind of test can I run to see a value in it?

Thank you.