A320 ADR 2 Fault
I think it's a REQUIREMENT rather than a recommendation.
When the ADR 2 pb-sw is set to OFF, and if the aircraft is in CONF 1+F, the SFCSn°2 Flap channel is unable to crosscheck the airspeed coming from the ADR 1 with the ADR 2 airspeed coming from the SFCS n°2 Slat channel. This may lead to an uncommanded flap retraction during the takeoff. As a consequence, takeoff in CONF 1+F is not permitted when the ADR 2 pb-sw is set to OFF.