In Title 24, Redline refers to minimum operating reserve that must be maintained at the power generating and transmission system. If you go below that line, it is a stage 3 emergency load shed, and instead of changing set-points, you have to shut the system off.
On the SSE control, this input is a network command that will set a 5 minute timer and turn off and lockout 50% of the capacity. On a 2 compressor unit, this would be compressor number 2. On a 3 compressor unit, this would be compressors 2 and 3. On a 4 compressor unit, this would be compressors 3 and 4. Each time a Redline flag is received (toggled), it will restart the 5 minute timer. When the timer expires, the control will resume normal operation.