An value that identifies the Primary Reference Source that
the network clock available from this source is traceable to
if configured as a source of network clock for NCDP.
The object takes the value 'internal' when the PRS for this
source is an onboard oscillator local to the device. For any
other cases the value 'external' is used.
If the value of this object is used as a tiebreaker by the
algorithm,'external' wins over 'internal'.
All 'external' sources of network clock are assumed to be
traceable to the same PRS by the NCDP protocol entity.