Originally Posted by Enova

While I'd encourage your friend to do his own research, I'm going to go ahead and explain the rotation. Basically, from left to right, this is a priority queue for a Marksmanship spec. When one or more shots are available, the one closest to the left of the queue takes prior

SpS is Serpent Sting, that you only need to use once per fight, unless the debuff falls off
CS is Chimera Shot, meaning that firing this is first priority if you have the Sting Debuff up (especially because but not limited to the fact that it renews the duration of SpS)
KS is Kill Shot - that only becomes active when your target is below 20%, but when it does come up, fire it every time it's off cooldown, unless CS is available
AS is Arcane Shot
AimS is Aimed Shot
SS is Steady Shot