Difference between cast off and decrease?

I just finished the back of my sweater. Directions for armholes were cast off 3 sts then 2 sts every other row, which I did. I am now doing the left front and the directions say decrease every other row 3 sts and 2sts. Do I use cast off or decrease (k2tog). Why the different terms? Confused.
Ps
The front neck edge also says dec 15 sts.at left edge. The back said cast off middle sts.
Is there a reason to do it differently on the front?
Thanks.

If you look at how a sweater is built, the decreases make a piece “lean” and get narrower, while the cast-off stitches leave a definite edge. The first cast-off stitches make the armpit. From there, you decrease, which makes the sweater front go up to the shoulder at an angle.

In the back, you’re casting off the middle stitches because they’re the neckline. You want that to be a fairly straight, level edge. If you tried to do that with decreases, you would have a big V-shaped hole in the middle back of the sweater.

Thanks Becky!
So if I’m understanding you correctly, for left front neck edge, when it says decrease 15 sts, (I should initially bind off) then it says dec 4 sts, then 2sts, then 1 st twice. (I should decrease, k2 tog 4 times etc.) every other row?

If you need to decrease 4 stitches, yes, you can k2tog four times, etc. Some people will start the decreases a stitch or two from the start of the row to make a smoother edge. Also, sometimes you’ll want a right-leaning decrease and sometimes you might want a left, so you might want a slip, slip, knit in some places if it looks better to you. It’s not going to ruin your project if you don’t, but it does look nice to have them lean the way you need.

What are the exact directions for the left front? If it’s decrease 4sts across the row, I agree, space out the k2tog and ssk. If its a stepped decrease at the neck edge then I would think it’s bind off 15sts at the beginning of the WS row, work a row, bind off 4sts, work a row, etc.

It is a stepped decrease at neck edge. The pattern says, “dec at left edge: 1x15 sts, 1x4sts, 1x2sts, and 2x1st.” I have 35 sts on needle, dec every other row would bring me to 12 sts which is what I bound off for back shoulder (2x6 sts.) The only part that is confusing to me is the back directions stated cast off for all dec at armhole and neck edge and the front says dec for all dec at armhole and neck edge. I also used the sloped bind off method as directed. Thoughts?

The sloped bind off is a very nice technique to use. I would do that for all the decreases except the 2x1st. There I would do a k2tog or ssk depending on how you would like these sts to slant, as Becky suggested.

Thanks Salmonmac,
That is what I am going to do.

Thank you to Becky also for your insight.
It’s great to get help from fellow knitters.