As just mentioned, the DCO has a modulator that mixes two adjacent DCO frequencies. Looking at the DCO clock on an oscilloscope, there will be a clock signal that contains two discrete frequencies. The mixing of the two frequencies provides a couple of benefits: 1) It allows a finer control over the effective frequency that the DCO clock provides and 2) It provides for a lower EMI device. The MSP430 has very low EMI emissions due to the mixing of the two frequencies.