At the moment unit selection is limited to the horizontal plane and around 6 units. Scrolling enables access to other units, which are unsorted.
It’s really inadequate!
Add the vertical plane to unit selection.
A1. Left click on a unit icon to select it for deployment.
B1. Hold left mouse button on a unit icon.
B2. Move the mouse pointer into the main screen area above the unit selector.
B3. A vertical unit selection will appear, according to the filter on that button (e.g. Medium size turrets).
B4. If there are too many units of that type to display, a vertical scrollbar will be present for scrolling.
B5. Release the left mouse button. If the pointer is on a unit icon, that unit will be selected for deployment and it will also appear in the horizontal bar for fast future selection.
B6. If the pointer is elsewhere, the vertical bar will stay until a unit icon is left clicked or the mouse pointer is left clicked in the main game area (in order to cancel the unit icon selection).
Filters could be game preset (e.g. Light Turrets, Medium Turrets, Heavy Turrets, Infantry). Or player defined (e.g. Expensive, Cheap, Favourite).