The only way to estimate distance to a beacon is based on measuring it's signal strength. However, the Received Signal Strength Indicator (RSSI) value alone does not mean anything, because beacons can broadcast their advertising packets with different TX power. It means that e.g. a smartphone should report RSSIs in different ranges for two beacons located in the same distance from a user, but broadcasting with different TX power levels.
In order to address this issue beacons in their advertising packets, in addition to payload with identifiers, also broadcast reference RSSI values. For iBeacon it's the RSSI at 1 meter and for Eddystone it's the RSSI at 0 meter. This way an application that tries to estimate a distance to a beacon knows what RSSI it should receive at a certain distance to a beacon. Then, using it together with the actual RSSI, apps can extrapolate a possible distance to a beacon.
Originally Kontakt.io provided only beacons with precalibrated RSSI@1m and RSSI@0m values that were suitable for most use cases. However, in order to accommodate more divers deployments we've recently introduced the Reference RSSI Value Calibration.
Right now everyone can do their own measurements and provide each beacon with firmware 4.0 or newer as well as Beacon Pros with custom sets of Reference RSSI values. This parameter is now configurable just like any other setting in the Web Panel as well as via our API.