A tubing string assembly is disclosed for fluid treatment of a wellbore The tubing string can be used for staged wellbore fluid treatment where a selected segment of the wellbore is treated, while other segments are sealed off The tubing string can also be used where a ported tubing string is required to be run-m in a pressure tight condition and later is needed to be in an open-port condition A sliding sleeve in a tubular has a driver selected to be acted upon by an inner bore conveyed actuating device, the driver drives the generation of a ball stop on the sleeve.