And how does it work? Because mine dropped to B and I have no idea why. In itself it’s not much of an issue, but it also dragged my overall rating down to B, which is making my life a little inconvenient at times, so I’d like to know what to do to get it back to where it should be. 
Hey! I just saw that this post seems to have slipped through the cracks. Sorry about that. 
Unfortunately, the exact workings of the ratings cannot be revealed in order to avoid meddling. On the bright side, I just checked your Activity rating, and it seems to have gone back up to A. 
Just found this old post (when I was about to ask about the topic) and I assume the same policy also applies to Stability and Reliability.
But isn’t the “we have no idea what these mean” factor effectively making everyone ignore them entirely? Wouldn’t they be more useful if we knew what they measured? Every time I’ve heard these asked about the person responding says everyone ignores them.
We know what they mean, we just don’t know how exactly they are calculated… except for Stability, of which we know nothing at all.
We know Reliability measures if you breach contracts often.
We know Activity measures how active you are in-game.
I very much doubt anyone told you they ignore Reliability. They ignore the others because they are irrelevant to gameplay, not because they are opaque.
That’s true, I was thinking of Stability when I wrote that. For the other two that’s probably good enough.