From D'addario:

XT Classical strings combine our infamous Pro-Arte treble strings and silver plated wound Composite basses with an extended lifespan treatment, preserving the natural tone and feel of uncoated strings. Available in normal, hard, and extra-hard tensions. 

D’Addario XT strings offer extended lifespan while maintaining the tone and feel of your favorite uncoated string. By utilizing the proprietary Composite basses and precisely intonated trebles of our legendary Pro-Arte strings, D’Addario XT delivers dynamic response and long-lasting performance. These are strings made to stay, for players who refuse to fade away.

• XT Classical strings feature an extended lifespan treatment on every wound string in the set, preserving the natural tone and feel of uncoated strings.-
• XT Classical string sets utilize the proprietary Composite basses and the precisely intonated trebles of D’Addario’s legendary Pro-Arté strings, combining dynamic responsiveness with long-lasting performance.
• 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.