What is the difference between directs and shortcuts in the Route Availability Document?
A shortcut is a deviation from the filed flightplan given to an aircraft by ATC during the flight, if traffic and other factors permit. A flight crew cannot expect to get any shortcuts, so they cannot plan for it.

Routes in the RAD are flightplannable routes. If a direct route is available in the RAD, it can be used when filing the flightplan, prior to the flight. This allows airlines to plan for the shorter route, thus allowing them to bring less fuel and advertise a shorter flight time.