Hi,
Those are internal connections to the DAC module.
The following will be in the next revision of the RM:
For this device, external voltage supplied to VREFH_ADC pin is the only reference voltage for DAC reference.
User will need to set up DACx_STATCTRL [DACRFS]=1 to select the valid VREFH_ADC reference.
When DACx_STATCTRL [DACRFS]=0, the DAC reference is connected to an internal ground node and is not a valid voltage reference.
-Anthony