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.
You can bypass 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