Key dispatch missions are (offline) dispatch missions that have a yellow dot next to them, in the Combat Deployment menu. Many of the first ones only require troops, but eventually you start needing vehicles for them. These ones will have a picture of the vehicle you need. I believe the first one needing a vehicle needs a truck.
The last 5 or so dispatch missions require the
These ones also require Walker Gears. Most of these have a really bad success rate. I did most of them with A++ combat units and they only had a 55ish percent chance. With S ranks it was only like 65%. Because of this, I would wait until late game to do these, or at least only ever use your worst people on them.
Another thing of note is that the later dispatch missions take 2 or more hours. These hours only count down when you're actually on the game, so failing can make this rather tedious. This is why I went with the A++, because they were my worst people but I still had over a 50% chance.
Comment from the games masta - If you abandon a mission, you do not lose your
but it will remain unavailable until the mission SHOULD have finished. All Walker Gears are lost.