From D'Addario:

XL Nickel are the industry standard in electric bass strings. With their clear fundamentals, booming tight lows, and reliable D’Addario construction, they’re trusted by players everywhere to handle any performance. XL Nickel bass strings are made with a high carbon steel core and wound with nickel-plated steel wrap wire, for consistently bright tone and smooth playability, great for a wide variety of bass styles. This 45-100 Regular Light set is the medium scale version of our most popular bass gauge, providing a perfect blend of response and playability.