ForWhat,
Neg or pos pressure are just ways to say how your intake/exhaust work relative to each other.
If your in and out are both active and move the same CFMs, you are neutral.
If your in moves more CFMs than your out, including if you have a passive exhaust (NOT recommended), you are positive.
If you have active exhuast and lame or no active intake, you are negative.
Got it? Good!