Witnesses on the Helium network are Hotspots that have seen (or witnessed) a Proof-of-Coverage packet from a Hotspot. This single-stage Proof-of-Coverage challenge is also known as a “Beacon”.
Witness lists used to be used to construct multi-hop challenges, but since the network moved to use single-hop challenges (or beacons), it is no longer used and only exists for informational purposes only.
Witnesses are informational and based on a rolling 5-day period of successful Proof-of-Coverage witness receipts. The list also resets if a Hotspot updates location, changes antenna, or elevation.