FYI - I'll be the Targan to Twilight 2013 here
So far what i've seen in the Twilight 2013 rules is tracer ammo:
Increases 'threat level' for those that you are shooting at if you're trying to suppress an area. Seeing the bullets overhead as well as being shot at.
This comes into play for the CUF (coolness under fire) and you can supress the enemy and the enemy can suppress you - causing inaction or 'breaking'.
Also....
Using tracer ammo in consecutive attacks increases the chance of sucess by a little bit for each round you shoot at the same target using tracer ammo.
I think they are both Stage III rules