Are they Visible or Not?

[Rabbit]

Nope, the leader is considered just another unit when it comes to concealment. Think of it in terms of if you wanted to move the units from one hex to another. You can move just the leader, just the squad of six men, and/or just the squad of nine men. They are all distinct units. If any one of those distinctly separate units fires a weapon, then that distinctly separate unit becomes visible.

In other words, if only the leader fires, then only the leader becomes visible (and the two squads remain concealed). If any member of the squad of six men fires, then the squad of six men becomes visible (and the leader and the squad of nine men remain concealed). If any member of the squad of nine men fires, then the squad of nine men becomes visible (and the leader and the squad of six men remain concealed). If there was no leader in the hex and a member of the squad of six men fires, then the squad of six men becomes visible (and the squad of nine men remain concealed).

And note, the same thing applies to movement. If any one of those distinctly separate units moves (e.g., moves from one hex to another or changes into or out of Ground mode), then that distinctly separate unit becomes visible. If the other units in the hex don't move, then they don't become visible.

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It looks like Abram beat me to the punch there. Darn his speedy internet connection! :)