The symbol Ø refers to the Diameter of a circle, or a circular arc.

R is for radius.

https://www.designtechacademy.com/2017/09/3-gd-symbols-diameter-radius-controlled.html
I see CR, SR, and SØ Could be used with a Leader.

I never took a drafting class. Is there a good, simple website for the basics?

- Brian

alt 0216 = Ø
(use numeric keypad)