Usually, after posting the Sales Orders/Purchase Orders, these documents are automatically deleted. In other words, once the Purchase Orders/Sales Orders are completely received/shipped & invoiced, these documents are deleted automatically.
Then, why I have a few Sales Order/Purchase Order still showing up in the system even if they are "completely shipped/received and invoiced"?
Solution (rather Explanation)
The Purchase Orders/Sales Orders are automatically deleted only when it is posted (completely received/shipped & Invoiced) from the document itself using the Post action on the ribbon Bar.