"Tip up" and "tip down" refer to the orientation of the folded knife when clipped in the pocket.
On a "tip up" knife, the clip is attached to the rear of the handle, so that the pivot end hangs downward, with the tip of the folded blade pointing upward.
A "tip down" knife is the reverse: the clip is attached at the pivot end of the handle, with the butt of the handle hanging downward in the pocket, and the tip of the folded blade pointing downward as well.
Most people around here seem to prefer "tip up", feeling that it's faster to draw and open; others prefer "tip down", feeling that the blade tip is less likely to catch the pocket, seam or...er... one's dignity, when drawing the knife.
Personally, I use both, and don't really have a preference.