They used the same number system as the Quadra. There was a Centris 610 and a Quadra 610. The Centris was a lower spec version of the Quadra. There was also a Centris 650 and a Quadra 650. Same deal. The Centrises used LC040's while Quadras used the full 040. Centrises usually shipped with less main memory and VRAM as well. The Centris name was dropped shortly after because the customers Apple was targeting with the Centris name were going either for something cheaper in the Performa range or spent the extra for a faster Quadra. There wasn't enough room in the marketplace for a range that split the difference in price and performance. Apple made more money by selling Quadras than Centrises, anyway. The Centris was Apple's Edsel.
I'm not sure of the logic behind the numbering system, but there is a heirarchy that comes into play. 5xx models are all in ones designed mainly for education, 6xx models are typically 5xx models without the built in monitor, 7xx is low end servers (the sole representative being the Q700), 8xx is mid range servers and 9xx is high end servers. Some models don't fit neatly into their appointed heirarchy like the AV models, which are a lot more powerful at some tasks than the other models in their number range. Some models numbers are logical (840AV, mid range server, 40mhz CPU and AV circuitry) but others aren't. (660AV?? there are no 60mhz CPU's back then). The 950 also doesn't follow since there also were no 50mhz CPU's yet, either. And what about the 700, 800 and 900? You could say as the first representatives (only in the case of the 700) in their classes that they used 00 at the end to show that, but where then is the 500? The 5xx and 6xx numbering schemes defy logic even further with a multitude of machines in a multitude of configurations, none of which can be adequately reflected in the numerical designation. Why were there 4xx machines and why does the Performa 200 stand out as the only representative of it's numerical group? Why is the Performa 600 the only 6xx machine to still use a 68030? Was there even a need to market the Classic II as a Performa? I'm sure someone has a list from inside Apple that shows the logic used in the numbering scheme, but I sure haven't seen it.