RSSI / Signal Strength

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At least for captures done on a personal computer (Windows + Npcap, Windows + AirPcap, Linux, macOS, *BSD), the signal strength is provided by the driver for the Wi-Fi adapter, so, for received packets, it's in terms of the wireless client.

It should be in terms of the wireless client everywhere else; it's easy to know what the power is as received, but harder to know how it was when it was transmitted if the host capturing the traffic isn't the host sending the packet.