From D'Addario:

XT Acoustic Phosphor Bronze strings combine our most popular acoustic alloy with an extended lifespan treatment on every string in the set, preserving the natural tone and feel of uncoated strings. Available for 6 and 12-string acoustic guitar ranging from extra-light to medium gauges to suit a variety of playing styles and preferences. 

D'Addario XT strings feature high carbon steel cores with an extended lifespan treatment on every string in the set, giving you enhanced break resistance, pitch stability, and long-lasting performance, all while preserving the tone and feel of your favorite uncoated strings. These strings are made to stay, for performance that never fades away.

• XT strings feature an extended lifespan treatment on every string in the set, preserving the natural tone and feel of uncoated strings.
• High carbon steel wire and Fusion Twist technologies deliver unrivaled tuning stability and break resistance. Strings stay in tune 131% better than traditional strings!
• Strings last at least 4 times longer than traditional uncoated strings.
• The inner packaging features a re-sealable zip closure, for ease of opening as well as storage of unused strings.
• Every set includes a Players Circle code for product identification, registration, and authentication. Players can collect points to get rewarded with D'Addario products and merchandise