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 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.