She does, though it doesn't really explain her callsign, since pretty much everybody does. The standard AEF aviation ammunition loading for full-auto weapons large-caliber enough to fire explosive rounds is what they call a "two to two to one" mix of armor-piercing, armor-piercing explosive, and armor-piercing tracer: AP, APX, AP, APX, APT, repeat. Lighter MGs that don't have APX available are loaded 4:1 AP/APT. It's also customary for the last ten rounds on a belt to be tracers, so that the operator has a visual indication that she's almost out of ammo.

Tracer is presumably called Tracer because she herself leaves a luminous trail when she uses one of her favorite Power Stunts,* which is to accelerate her personal time for a moment so that she appears to teleport a short distance.

--G.
* as specific applications of superpowers were called in the old Marvel Super Heroes RPG
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