Difference between m1 and cable cast on

I am knitting a sweater in the round…started with neckline and I am coming to the sleeves. I am not working with a pattern…just from a picture of a loose knit (size 20 needles) throw-on sweater.
I am coming to the sleeve part and I plan to cast off 20 sts on each side for the sleeves. But do I cable cast on extra sts or knit into front and back to make room to complete the body. The sweater looks like this, sorta
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KFB doesn’t work that way. It’s an increase for within a project. Use the cable cast on, knit on, or backward loop for those stitches under the arm.

pic

You need to cast on stitches rather than just do an inc. An increase is for adding 2 or 3 sts in one place, while a cast on is for more sts to span a space.