1. R in number theory represents the real number set in set theory, and the real number part in complex number is also represented by this symbol.

2. Geometry r or r represents the radius of a circle.

3. In geometry, ∠R stands for right angle.

4. Geometry r also represents radian (an angle indicates that 360 degrees equals radian).

5. Calculus uses the capital R of writing style to express Riemann integral.

So, what you said, 2R, roughly represents the diameter d in geometry.