SITL architecture

This link:

http://ardupilot.org/dev/docs/sitl-simulator-software-in-the-loop.html

has a great diagram of the sitl archtecture. One question I’ve been trying to figure out is what the extra link (the UDP port 5501) between the sitl and mavproxy is for. Can anyone shed some light on that and maybe update that page to make that more clear?

From the code I’ve looked though, it looks like maybe RC commands? Is that right?

Yes it is for RC input see https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_SITL/SITL_cmdline.cpp#L132