Driver "move out" command comes if you are gunner but not the owner of the vehicle.
It also comes if someone is sitting in drivers position.
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If no one is in drivers position and if you are owner of tank. When you want it to move it moves.
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To be owner of a tank you need to be in commander position when multiplayer mission begins.
To be owner of a platoon, you need to be commander of first vehicle in the platoon. Usually 1/1
To be commander of company you need to be CO. Or what ever vehicle is most top on the vehicle list in joining screen.
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If you get into situation where you have no control, it's likely that you weren't in commander position at the start, and in that case who ever is owner of your desired vehicle. Should right click that. Vehicle and option give to "your name"