I have not used my ATRAC, I use Multi/Terrain and think of it as a variable ATRAC controlling the amount of slip.
I set it for Sand and use it on sand, loose dirt and snow without the locker. It works well in snow. I can get all wheels spinning and it just takes off.
The normal traction control will limit engine speed and bog down, but M/T on my 2013 keeps going. When I use the the locker I set it to Rock but not sure it matters with 3 of the 4 wheels already locked together.
Get out on something slippery and try the settings to see what they do to help you decide which to use. You can have fun doing this part.