Azwan.

iirc the 10 different circulator items have different chances of improving the rank of your inner ability when they shuffle it around, much like cubing.
So basically the more you spend on your circulator the better the odds of rolling good lines.
Not sure if it's possible to go down ranks or not.

It is entirely possible to go down ranks when using any Circulator (unless you have bought a Cash Shop Inner Ability Lock, which by the way only exists for B and A ranks, lol). Well except for the S Rank Circulator that they gave out in a Hot Time, that instantly brought your Inner Ability to S Rank.

Speaking of which, if you do get that, I highly suggest getting all three lines before using it because it will make all of them S Rank iirc. And don't reset them, they'll most likely go back to C Rank.