"Custom formations" and "move in formation" are two different things, and they probably don't play well together. Custom formations is click-and-drag while issuing a movement order; move in formation is control-click while issuing a movement order. I'm honestly not sure what happens if you try to control-click while clicking-and-dragging.
Click-and-drag gives each unit in the selection a different destination, spaced out evenly along the line or curve that you draw. Each unit then moves towards its assigned destination at its own pace. You can't control which unit goes to which location, and you can't control what order they arrive in. It's probably most useful for a homogeneous group of units.
Control-click gives each unit in the selection a different destination, but each destination is based on their current relative position, so that they formation they end up in is the same as the formation they start in. They also move at the same speed, so they all arrive at the destination at the same time and stay in formation throughout the entire move.
So you might want to, for example, use click-drag to send a group of assault units into a line, then again to put a group of skirmishers in a line behind them, then again for a line of artillery behind that. And then use control-click to move the entire squad into battle while staying in their three assigned ranks.
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I checked out these things which I thought would mentioned ctrl-click, but they don't seem toIt actually is mentioned on the
Using Orders page:
"Holding down on ctrl while giving a move order issues a "Move in formation" command. In this case, formation means whatever formation or configuration the units were in when they were given the order." But it's easy to miss. There's a lot of stuff on that page.
