The new Mission Editor is an evolution of the original "FSX Mission Editor", combined with many of the ideas used in the Dovetail Games "Pro Mission Tool" for Flight Sim World. It allows you to easily design and test your own missions, leading you from simple beginnings to complex, evolving situations for fun, training or storytelling.
Use the free trial version to check the tool out and even use it to create entire small missions.
Included in the package is the SimVar mission system extension, giving many additional commands to mission builders. These commands were the ones added to Flight Sim World and make your design job easier.