According to Hilbert's axioms, point U is considered to be between points C and D.
Hilbert's axioms are a set of 20 assumptions which were proposed by David Hilbert in 1899.
It states that;
For every two points, A and B there exist a line a that contains them both. We can, therefore, write AB = a or BA = a. Also, If a point B lies between points A and C, B is also between C and A.