Wool is a very widespread material. There are more than 200 different sheep types exists around the world. In the wool industry you measure the wool in three main different categories:
More the wool fibers are thin, more the wool is soft and cozy. Merino which is a very thin wool (18 - 21 microns) is largely used in appeal wear. But Merino wool is hardly used in the footwear wool industry. There is a lot of friction and the shoe would run out in a couple of months. Furthermore, Merino wool is extremely expensive. For a pair of shoes you need 5 x 6 more wool than you need in appeal wear. Would you pay 300 Euros for a pair of slippers ?
Here at Baabuk we used in the 30-32 microns thickness. Your feet will just love the wear the slippers barefoot in and around the house.
There are many more different wool types used in different products. Check out this very interesting explanation.
Courtesy: Words from the WaterTower