Why: With how many buildings and units there are it is often hard to select only the units that you want. Especially when they involve multiple types of unit and Ctrl+LMB doesn't cut it. This is particularly most useful when dealing with lots of drones.
How:
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Option 1: Add something similar to Ctrl+Drag Select to ignore buildings in the selection.
Option 2: Use Ctrl+LMB on a unit icon on the list of selected units on the left of the screen to isolate only the units of that type within the current selection.
Option 3: Add filters to the selection, like it is done when searching for units within the control panel screen.
