That’s the sensitivity voltages. The PE-3 has not been used for years. It was replaced by the PE-11 and DB-11 Base. It is a conventional 24vdc smoke detector. If you need a .pdf on this let me know. Cerberus/Pyrotronics was purchased by Siemens in 1999. Most of the older products were replaced by low profile detectors. The PE-3 only lit up the LED red if it initiated an alarm. The replacement detector, PE-11 has a tricolor LED. Green flashing for Normal, Yellow flashing for Trouble, RED flashing for ALARM.