Well the people that did the webpage for Trek incorrectly used the term "seat mast" and it is spreading to the bike shops. A true seat mast clamps onto the seat tube. A seat post inserts into the seat tube. Trek has a seat post, not a seat mast.

For example, the Scott Addict frame accepts a seat mast.
http://www.cyclingnews.com/photos/20...ated_seat_post