If you mix readings from the version 2 user guide (where class filenames were lower-case) with readings from the current version 3 userguide (where class filesnames are UCfirst), you will find conflicting information!
Use the user guide for the version of CodeIgniter you have installed, and there should be no confusion.
There could still be confusion searching online, as outdated webpages may still refer to version 2, which is now alegacy version and no longer supported.
