From my understanding R is just a variable which represents the number components of the vector RightVector, UpVector, and LookVector components of a CFrame.
These numbers do not have units as these direction vectors are unitized (length of one). Just like a vector with (0, 1, 2) has the same direction as (0, 2, 4) what matters is these numbers combined such as the right vector which is (R00, R10, R20).
The 00 and 01 notation is just matrix row x column notation.