the cross attachment screen is for formations, so e.g. you can add a mortar section formation you bought separately to a rifle company's command (instead of leaving under the bn commander which is the default).
The cross atach BUTTON is for individual units that are not the platoon command of a platoon, to allow you to put these under the control of another platoon. E.g, you may put the CHQ MGs under command of one of the rifle platoons and leave the mortars under CHQ command. This button is made available in the deployment screen for generated battles and campaigns (not scenarios, the scenario designer has done that for you
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Cheers
Andy