This Gauge reports the number of times this line entered AIS alarm state during one specific historical 15-minute interval in the past 24 hours. An admin cares because a nonzero historical FC at a specific interval identifies exactly when the line was flapping into and out of alarm, which combined with adjacent intervals' FC values shows whether it was one brief flap or repeated instability during that window. For example, an FC of 4 within a single 15-minute interval indicates the line dropped into and recovered from AIS alarm four separate times in just those 15 minutes.