The documents where you can add assets are all those belonging to the SAT or Maintenance Module and invoices: Incident, Quote / Work quotations, Purchase order / Work order, Delivery note / Work note, Invoice.
To add an asset to a document you have to enter the document and tab "Invoice".
To add an asset to a document you must enter the document and in the More information tab, field "Assets", you must click to display all assets related to the customer and you can choose one or several.