It is coming up on winter and one may notice that the back vent of that brand new winter coat has an X losely sewn at the bottom...

Cut it out! 

Often garments are shipped flat in boxes over long distances. This X, or stay stitch, is in place to ensure that the vent does not flip up and over during its long journey. Flippage would create a very nasty crease, given the inevitable changes in temperature and humidity, which would take all kinds of work to get out.

Same goes for welt pockets and pleats, remove the stay stitch before wear.

Pictured above is an illustration of vents and a little definition; and below, a diagram. Pretend that it's the back of that brand new winter coat...