If you see a lock beside an item on functions, event-wide details, or your rental rates, the item or rental rate has been invoiced - it's been added to an invoice and the invoice has been sent to the client. Since the item or rental rate has already been invoiced, it cannot be edited. If additional quantities of this item need to be billed (such as for usage on the day), we recommend adding the item to the function again with the additional amounts.
If the invoice is in error and needs to be updated and re-sent to the client, it can be marked as un-sent so that the items and rental rates can be edited. Only users with the Mark Invoices Unsent permission are able to mark an invoice as un-sent.