Explanation of the Monitor Port Output Specification on the AQ2200-311A

The meaning of "Monitor port: -13 dB" is that -13 dB corresponds to about 5% (10Log(0.05) nearly equals to -13). That is, output power is divided by an optical coupler at a ratio of 95:5, and 5% is for monitor output.
  

If you measure initial power level as a standard value, you can get the optical power level at the main port by measuring the attenuation at a monitor port because output power is attenuated as the same ratio at either a monitor port or a main port.