What does this do?

Can anyone explain to me what this feature is on this old Abu Garcia 4600 Plus reel?

I’m asking about the sliding button that you see on the left hand side of the photo. It is numbered 1 through 10, and there is a black button at the bottom that can be set to one of the 1 through 10 positions. What the heck does this do?

I haven’t seen this on other reels and my Google searches for a manual or other explanation hasn’t produced anything.

Thanks gang !