big brother

Purchased an OEM radio for 98 Civic LX from ebay recently identical to
the link below:

http://tinyurl.com/lqbn8

Can you help identify the part viewed from the picture of the backside
of the radio..It shows a green connector with a black circle to the left
of it..

The tag on the part identifies it as a "Security Control Main Unit" with
a slide switch for either Cx/Dx or Hx/Lx/Ex..

My FSM doesn't indicate what this is for .

What is it and what is its' function?

Thanks,

Rich