The coverage overage area heatmap shows the span of the signal (in terms of area) and intensity of the signal. The RSSI cutoff is the lowest value of RSSI in dBM which is considered a usable signal and it is shown in dark blue color. RSSI cutoff value in rendering heat map, identifies the span over which the signal is higher than the specified RSSI cutoff value.
https://www.cisco.com/c/en/us/td/docs/net_mgmt/prime/infrastructure/3-
10/user/guide/ciscoprimeinfrastructure_3_10_userguide/use_wireless_site_maps.html#concept_0
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