The main distinction of Clark's "Un.", or "Unstructured", series is the air circulation system that is built into the shoes.  I am one of those whose feet sweat a lot and my feet can get uncomfortably hot in some normal shoes.  Although somewhat skeptical about the product claims I bought a pair of Un. Coils and Un. Dentons at the same time:  I am very favorably impressed.  I would say these shoes have reduced the amount of moisture retained by my socks by about 50%.  The shoes are very comfortable, soft, flexible, and seem to be very well made.  I am quite happy and am now a big fan of the "Un." series.