Swing weight is a number that really has nothing to do weight, but more the perception of weight to a club, mostly for feel. The scales run from A-F and 0-9, A0 the lightest and F9 the heaviest. A D3 will feel heavier then a C9, regardless of the actual overall weight of the club, most womens clubs are in the C weight range and mens in the D range. By moving the balance point of a club will effect the SW of the club. For example if you have a club that swing weight on a swing weight scale to D5 and that feels heavy, you can add weight to the butt of the club, and even though you made the club physically heavier the swing weight will be lower, 3 grams would probably reduce it to D3 or so (just an example), but the club will feel much lighter when you swing it. The reverse in the other direction. Going much into the A, B, or E ranges are extremes, I don't recommend A or B ranges except for kids, and E ranges only for the very strong players. All of this has to do with balance points and the relationship of the grip end to the head end of the club.

My personal preference is to have all my irons at D2 except my wedges, I set to D5 and my sand wedge to D9, my woods are D4. Why do I have it that way, who knows, it just feels good, except the sand wedge, there is a real reason for that, it slows me down and gets through the sand better at the heavier feel with the added weight in the head, my sand wedge is also cut shorter, standard length shaft is 36 inch, mine is 34 inches.