Market directional

Page 167 of volume 4 states: “For this reason, many investors consider mortages to be market-directional” --> what’s the exact meaning of market directional?

http://www.analystforum.com/phorums/read.php?13,1097048,1097052#msg-1097052

It’s a weird term to use to describe it. I would have used something like ‘market-skewed’ or something. ‘Market-directional’ sounds like it moves the same direction equally.