Smart Invoices allows you to see the status of your Invoice. Which means you can see if the Invoice has been emailed, viewed, paid or deposited all from the QBO Invoice Smart Dashboard. No longer will you need to wonder in what stage the invoice is, rather now you can view the status at a glance. You can also click into the Invoice to see the activity stamp of the all views including when it was last opened.

To read more about Smart Invoices in QuickBooks Online you can follow the link.
http://www.intuitiveaccountant.com/general-ledger/all-new-quickbooks-online-smart-invoice/#.WoXveainHcs