The player decides the order in which his stacks try to move. A stack that failed to move can not try a gain in that player's round.
The order in which a player tries to move his stacks has a certain importance. E.g. A stack might have to move into combat with a stronger stack, when it moves. But the stack must only be moved, if no other stack passes a command test. So that endangered stack will try last, or not at all, if one of the other stacks manages to move.