The purpose of the build out line is to promote player development- allowing a team to play the ball out of the back in a pressure-free (or less pressured zone) at the start of goal kicks and when goal keepers have possession of the ball.

Once a goal kick has been called by the referee or once the goal keeper takes possession of the ball, the defending team (the team without the ball) should immediately run behind the build out line.

No player of the defending team may cross the build out line until the ball has been put back into play and the ball has left the penalty box area.  When the goal keeper is in possession of the ball, the goal keeper may put the ball in play at anytime and they do not have to wait until defenders are all behind the build out line first.

Finally, at the U10 division, the build out line, and not the halfway or center line, is used to denote offside violations.  In other words, an attacker cannot be penalized for offside in the zone between the halfway line and the build out line, only in the zone between the build out line and the end line (goal line.)  There is no offside called in U6/U8.