From D'Addario:

XT Nickel coated electric guitar strings offer enhanced break resistance, better pitch stability, and longer-lasting performance than standard electric strings. The XT coating, applied to each string in the set, delivers advanced corrosion resistance, while still retaining the bright tone and natural feel of the uncoated string. Beneath the coating, XT Nickel are made with a NY Steel core, nickel-plated steel wrap wire, and Fusion Twist technology, resulting in added resilience and tonal versatility, excellent for a wide range of musical styles. 9-42 Super Light gauge strings are easy to bend and very flexible.