sd_ble_gap_tx_power_set variables

[vn2000]

I am using nRF52 DK to evaluate the TX power settings using sd_ble_gap_tx_power_set command:

sd_ble_gap_tx_power_set (BLE_GAP_TX_POWER_ROLE_CONN ,  m_conn_handle, tx_power);

I'm still unclear about this last  tx_power variable:

What is the range of tx_power that I can set?

Can it be a negative number like (-20)?

What does tx_power correspond to the actual power like mW?

What is the default value of tx_power at startup?

[Kenneth]

You may find this useful:
https://ham.stackexchange.com/questions/1440/what-is-dbm-and-why-is-it-used-rather-than-watts 

[vn2000]

Thanks for the info.