In the Electrical Calculation document, it states "the CU (Coefficient of Utilization) value is calculated for each fixture using the RCR, the IES Data file, and the values specified for the space."
You may still wonder how this works. If you refer to a coefficient utilization table (such as that found here on Page 2). There are 4 major columns, the one one the right labeled 'Coefficient of Utilization'. For each fixture/lamp combination, a CU table is presented. These tables typically are created in a certified testing environment by or for the lighting manufacturer. On the left of the table is the RCR (0 - 10). At the top is pf, pc, and pw, which are floor reflectance, ceiling reflectance, and wall reflectance. Using the RCR and reflectances, you can manually look-up/interpolate the CU in the table provided on the fixture data sheet provided by the manufacturer. Or, if you happen to be software, you can look it up in the .ies file associated with the fixture.