how to use vehicle command for setting parameter?

Hello, 

I am using DDS control to send PX4 messages in SITL.

I have tried vehicle commands in SITL, and they work. For example, command: 176, param1: 217, param2: 1; these can change the mode to position mode.

However, I have no idea how to use command 180 to set system parameters. Could you provide a document or reference that I can study?
