Sherpa fur itself is not a standard term, but a commonly used name for businesses. The reason why it is called Sherpa fur is that it is artificial Sherpa fur, so it is called Sherpa fur, not lamb hair. Sherpa fur is not as expensive as lamb hair, but it has a good thermal effect, no less than lamb hair. So it is widely used in the clothing industry.
There is a difference between Sherpa fur and lamb hair. Lamb wool is really superfine wool, and selected long wool.