(Click on the screenshot to enlarge).

When ‘many’ purchase orders are joined, all orders are removed from the ‘Copy from’ account and moved to the ‘Copy to’ account.

Click on the ‘Remove purchase order’ button in the toolbar/ribbon to remove a purchase order.

Click on ‘Filter’ to create a filter which will find a purchase order. Read more about filters here.

Click on ‘Join’ to join the selected purchase orders.