CMC stands for calibration and measurement capability, while MU stands for measurement uncertainty. CMC is a calibration laboratory's best capability. This means calibration was performed using the best standards, with the smallest uncertainty, the smallest error, and under the best environmental conditions, with calibration performed on a very high-end piece of equipment. In other words, the calibration is as near ideal as possible for a given calibration laboratory. These CMC values are reported on your scope of accreditation.
Because a calibration laboratory cannot always work in its best environment and with its best reference standards, it needs to calculate an MU for each particular instrument it calibrates. In this case, the laboratory needs to estimate the uncertainty, taking into account potentially larger uncertainties and errors (compared w those in its CMCs) from its working standards, the contribution of more environmental variance, and possibly use of a lower-end piece of equipment with less resolution.