Selection

You can select your units with your mouse, with your keyboard or both.

Simple click, select what you click on Double click, select all same units/buildings on screen Box select, select all inside the box with highest selection rank

To manage your selection you can; Right click a unit in the selection window to remove it from the current selection. Shift right click will remove all units that are like the one you click on. Shift + selection command will add any thing to the currecnt selection

Selection Rank

Selection rank is what decides which units will be selected when doing a box drag over multiple unit types such as fighters, builders, buildings.
There are 4 selection ranks 0,1,2,3.
0 is for buildings 3 is for fighting units.
When doing a box drag over units with different selection ranks, only the units with the highest rank will be selected.
You can see a units selection rank as a large grey number on the side of the command menu, generally fighters are 3, builders are 2, buildings are 1. You can change a units default selection rank in the in game settings, for example commander set to 2 will avoid accidentaly cancel building queues or killing your com by issuing commands that were meant for your army.

You can bypass selection rank this by holding shift while dragging, you will select all units/buildings in the box.

Hotkeys

There are multiple Hotkeys to help with selection:

Z ctrl B ctrl C ctrl Z ctrl X ctrl I ctrl E ctrl A ctrl G ctrl F ctrl S ctrl T

factorys

silo missiles

autogroups

Autogroups

Autogroups are your custom hotkeys that you can set up on the go.
You can find how to do so on the Hotkeys page.