From the Game Guide just above
Artillery Bombardment Screens, including Air Unit Handling
Quote:
A
Selects the 'All Formation' mode. This is very useful if you wish to set the targeting range of an entire formation with the 'Y' key OR if you wish to move an entire formation to a specific point but it is VERY important to remember that you cannot undo a group move so Remember to toggle this off after the 'Y' operation, or the next time you try to move a unit, the entire formation will move simultaneously under AI control and you will be stuck with that move
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They don't actually move "simultaneously" but once you select that and click on the hex you want to move to they will all move towards that hex in sequence and there is no stopping or changing (undo) after they have moved there so you need to know where your units are and be careful with it.
It can be really handy for moving units down a road that is not in the enemy's LOS and it can be handy if find you have units from a formation spread too far away from each other and you can use "A" to select a point they can move towards to get the unit back together.
Build a simple test scenario first and observe how it works and experiment If you are far behind the lines you can get away with picking a distant hex to get them moving in that direction but if you do use it nearer the front only click on one hex in front of the lead unit. It can be a bit unpredicable and that is why short moves are best generally