This table are for those who are using the SR / XR series of cards.
Model Max Programmed Power Max Actual Power Power Offset SR2 16dBm 26dBm 10dB SR4 19dBm 26dBm 7dB SR5 19dBm 26dBm 7dB SR9 16dBm 28dBm 12dB XR2 18dBm 28dBm 10dB XR3 15dBm 25dBm 10dB XR5 18dBm 28dBm 10dB XR9 17dBm 27dBm 10dB
What the table means to you:

The "Power Offset" column represents the card's external amplifier, and it's power rating.

The "Max Actual Power" column represents the card's maximum transmit power.

The "Max Programmed Power" column represents the maximum number you should enter in the Atheros configuration dialog. This number will be added to the "Power Offset" column for the given card to produce the final output power. If you exceed this number, the card will be over powered causing noise, higher than normal processor usage (due to the extra noise), and possible packet loss not to mention shorten the life of your card.

For an XR9, a configuration dialog power setting of 15 will produce 25dBm of output power. (15 + 10)