LimeSDR VCTCXO control / External Reference

In the LimeSDR board the filtered charge pump output of the phase detector is connected to the Vc pin of the VCTCXO along with the DAC output voltage.
I am unable to understand this working. Is only one of them operational at any given time or are the voltages added (superposed simply by connecting them together?).
This seems to be a very interesting topology to lock to an arbitrary external reference (i.e., in this case the 30.72MHz TCXO can be locked to an external 10MHz reference). Am I correct in my understanding?
This would be different compared just switching between in the internal XO/TCXO/OCXO and an external reference.
Looking forward to your inputs,