The way to stabilize


I am new on these forum so hello everybody. I am interested in ArduCopter.

I just start the developper guide on this page :

I saw that the loop is made like this

But in the code the stabilisation is made like this:
Rate_mode(low level) -->fight_mode–>stabilize_mode

Why the loop is made like this?

Can someone help me about this?

Best regard.