To better understand what the pro's are talking about, I have inserted a picture of a "band joist" and given it's definition:
A "band joist" is a wooden joist perpendicular to the direction of the joists in a floor framing system, closing off the floor platform at the outside face of the building.
As you can see it is a good place for water to collect if it isn't protected properly. One of the worst areas for water collection is behind the deck band joist as the picture details.
