CE numbers?

[Janusxvii]

I've been using clearomizers for a few months now and I am loving them (especially with replaceable heads and an XXL ego twist battery). One thing I have noticed though is that a lot of vendors and ECF posts use what I am going to call CE numbers (CE2, CE3, etc...) but I cant find any any sort of reference as to what this stands for or means. Any chance a more experienced person can point me to where I can better educate myself on this subject?