Bluetooth power is expressed in Decibel-milliwatt (dBm), the unit used to measure radio frequency (RF) power level. For a Bluetooth device, 0dBm is the standard power level and +4dBm is just over a doubling of power. -3dBm will be half power, -6dBm will be a 1/4 power and -9dBm 1/8 power. Each change in ±3dBM is a doubling/halving of power.
Bluetooth Range and Power
KBA_BT_0405: Bluetooth Tx power settings
Transmission power, Range and RSSI
Choosing the Transmitted Power
What are the Estimated Distances for Tx Powers?
Bluetooth Tx power settings
Read about range.