CST - F-parameters for waveguide port as a monitor

Hello everyone,
I want to use a waveguide port as a monitor and a discrete port to excite the antenna. I get F-parameters once the simulation is finished, and I get two results: I2,1 and O2,1. Could someone please tell me which one I should look at and which one tells what? My second question is, are these F-parameters normalized to some reference signal, or do I need to normalize them?
Thank you.

CST Studio Suite