This read-write object masks or unmasks the external reference alarm: unmasked(0) or masked(1). This alarm fires when the unit loses its configured external frequency reference, set via cdm570ReferenceFrequency, so masking it only makes sense on a unit deliberately configured to use its internal reference, where an external-reference-loss alarm would be a permanent, meaningless nuisance rather than an actionable fault. A unit that was reconfigured from an external 10 MHz house reference back to its own internal reference, but where this alarm mask was never updated, could show a persistent external-reference alarm that no longer reflects any real condition -- a good reason to mask it once the internal-reference configuration is confirmed intentional.