This command sets the output power to be used on the next transmissions. Refer to
the LoRaWANTM Specification for the output power corresponding to the
and also to the RN2483 Low-Power Long-Range LoRaTM Technology Transceiver
Module Data Sheet (DS50002346) for the actual radio power capabilities.
In the LoRa spec (actually the regional parameters) the power index used in MAC commands is in 2 dB steps, so they might mean that.
The actual radio chip can’t exactly do that though, the actual power settings of the radio (hiding behind the firmware) are given in the mentioned hardware data sheet DS50002346.
If you don’t have an RF power meter, one way to profile what is going on is to measure the supply current during transmit at various settings, for example with a capacitor-filtered small resistor in the supply line and watch that with a scope. Or toss an INA219 on an Arduino and make yourself a logging power meter - tons of fun for looking at node power consumption in general though you probably need to change the default sense resistor on the Adafruit PCB or its clones.