Pattern Says to Mark Armholes

[Racerkatie]

ok, so I'm knitting along on the back of a little sweater ... stockinette. And the pattern says that once I've knit 4.75 inches from bottom I am to "mark" for the armholes. Then knit more until I have 9 inches. and at that point I do something else like work on the neckline or something, I forget. I'm looking on theories for what this means, why I would do it and how I would do it. It isn't like it says "k 10 and place marker" -- that I would understand. I'm stumped ... hence, any ideas?