CEU is lightweight GTCE addon which adds two types of buffer: CEU, and CEF. CEU converts GTEU to FE, CEF converts FE to GTEU.
CEU pushes FE on front side, receives EU on other sides. You can put FE batteries in CEU and use it as charger, or put EU batteries if you need some internal buffer. CEF does same things, but in reverse.
Shoving too much FE on CEF does not create any explosion. However CEU still can explode due to overvoltage.
Default conversion rate is 1 GTEU : 4 FE. You can change this at config file. Also you can turn off the recipes of CEU, or CEF, or both.
And from version 126.96.36.199, you'll get infinite energy source/energy void for both FE and GTEU.
and both of the name, CEU and CEF, has no meaning.