What does $this refer to in a Model?

By actually reading my second sentence and those links?

I'm not trying to offend you, but maybe you misunderstand my question.
I can surely use _get to get access to function and variable of CI_Controller. I have known that quite before.
But my question actually is, since I have experimented before, why can I use $this->cache->redis in a model, in which $this should refer to the Model Class, not the CI_Controller?