Garter and Seed Stitch Cardigan by Diane Zang

There are side vents in this pattern. After knitting the back and shaping the shoulders the pattern says, “Side Vents, With larger needles and RS facing, pick up 9 sts along long edge of side vent. Work even in garter st for 5 rows. Bind off loosely and do the same to other side.”

Looking at the picture on the front of the pattern, I’m not understanding what the long edge and the short edge is, and why I’m supposed to knit these “tabs.” Can I skip that part?

I don’t have the pattern.
The photo I found does not show the underarm.
One person posted about the vents on Ravelry and changed how they were done.

I suspect it is a gusset type thing under the arm, Omitting it may make the sleeve/armpit a little tight.
The long edge is simply the edge parallel to the side seam (where you are picking up the sts). The short edge would be perpendicular to the side seam and parrallel to the cardigan’s hem (its the width of the armpit).

Vents are usually the slits at the lower edge of the sweater or jacket similar to those shown in the picture. I suspect thare was a point after knitting about 5-6 inches in the back and fronts where you increased stitches. This notch is the vent. The long edge is at the ends of the rows. It’ll give a nice finishing touch to the bottom of the sweater if you pick up stitches along this edge but you can probably leave it out. See how it looks when you pin everything together and then decide.
http://www.ravelry.com/patterns/library/garter--seed-stitch-cardigan