why does my mouse cursor have a circle at the pointer

My magic mouse cursor changes shape without my knowing how. Clearly it's something I'm doing but I don't know what and can't seem to find out.

The arrow pointer currently has a circle at the point, this turns into a circle when it's an I bar and a circle at the end of the pointing finger. .

Are there control keys to change this, if so does anyone have a list, these functions may be useful. I'm still using 10.12.6 because I have important software that I don't think will function if I upgrade.

Thanks, Louise