622C-tx controller is for Canon cameras and E-TTL compatible. 622N-tx controller is for Nikon cameras and I-TTL compatible. I presume that the suffix 'C' and 'N' are representative of respective manufacturers. YN685 speedlight has got a Canon as well as a Nikon counterpart. Doesn't Yongnuo product line follow the suffix convention for all of its products?
The Canon Version of the YN685 was announced in the Summer of 2015 and began shipping in September of 2015. The Nikon version of the YN685 was not announced until March 23, 2016 and didn't hit the streets until May 2016. That may have something to do with the Canon version not having a "C" in the name as listed at most online retailers.
The official model names are YN685 C for Canon and YN685 N for Nikon as seen in the fine print in the listings at B&H. Amazon lists the Canon version as the YN685 and the Nikon Version as the YN685 N.
Not always. I think the C/N notation is as much of a seller convention to avoid returns :) as a Yongnuo production one. Or, it could be because Yongnuo typically reverse-engineers some bit of Canon gear, so the Canon version is released long before a Nikon version is. So for a long period of time, there may only be the single Canon version (e.g., the YN-568EXII was like this, too). They also do sell "universal" manual-only flashes, such as a YN-560III, where the same version works for both Canon and Nikon. They use the C/N notation as often as they use "for Canon" and "for Nikon" on their eBay listings.
The easiest way to distinguish if a seller listing is for the Nikon or Canon version is to hope that the image of the product is correct :), and to check the color of the lettering on the unit. For all TTL-capable Yongnuo flashes, if the lettering is silver, it's the Canon version; if the lettering is gold, it's the Nikon version. You can also check the pins on the feet of flash or triggers.