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This is particularly useful in the case of VTOLs - usually your VTOLs will keep attacking their last target until it's destroyed, the only way to stop them doing that is with DORDER_STOP.
It's not entirely clear what will actually happen to your droid when you tell it to "STOP" - the effect will probably depend on the type of droid and what it's currently doing. It's probably best to issue another command immediately afterwards (for example, DORDER_HOLD) so that you know for sure what your droid is doing.
Availability
Warzone 3.2 and above.
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