From D'Addario:

Born from our greatest breakthrough in string innovation, XS Phosphor Bronze strings have the longest life of any D’Addario acoustic guitar string. The XS coating gives our warm, balanced phosphor bronze strings maximum life, with an uncommonly smooth feel. Made with a NY Steel core, phosphor bronze wrap wire, and Fusion Twist technology, XS coated acoustic guitar strings have incomparable break strength and tuning stability, in addition to their dramatically increased lifespan. 13-56 Medium gauge strings provide maximum volume and projection, with more tension and resistance.