The 6-3 defensive front is similar to the 6-2 with a slight difference. There are 6 down linemen, with 3 linebackers, and 2 defensive backs. The 6 down linemen include 4 defensive tackles (2 nose guards) and 2 defensive ends playing outside of the offensive tackles. The 3 linebackers play inside of the defensive ends and they may include 3 linebackers or 2 linebackers with a safety playing up close to the line of scrimmage. This variation just depends on the personnel that the coach would like in the game and the situation.
The 6-3 defensive front is usually used to stuff the run especially around the goal line. Having 9 players close to the line of scrimmage allows much more penetration than when only 7 or 8 players are in the same space. Having 6 down linemen also forces the offense to adjust because if another player in the form of a tight end or running back is held to block, the defense has a free run to the quarterback. If a safety is lined up close to the line of scrimmage in a linebacker's position this could also bait the offense to throw over the middle where the safety can make a play on the ball.
This alignment cannot be used effectively in higher level football or anywhere not close to the goal line. It is difficult to cover the pass because the corners are left alone to play 1-on-1 with their wide receiver matchup and also because any receivers inside are able to be matched up on a linebacker or no one at all. Covering the pass however is not the goal of using a 6-3 defensive front as it means to stop the run and the offense close to the goal line as its disadvantages do not allow it to be used anywhere else.