Sateen could be cotton too, not man made material

It is just a way of weaving process. It gives a soft and silky feel and sheen some people like.

Sateen produces the sheen and softer feel through the use of a different structure in the weaving process.

Here are some cotton sateen I highly recommend:

http://www.garnethill.com/signoria-firenze-sateen-bedding/259595?defattrib=&defattribvalue=&listIndex=0