Compatibility section is in the user guide's style guide is for add-ons codes only?

[eluser]searain[/eluser]
http://ellislab.com/codeigniter/user-gui...mmon_words

Class and File Names using Common Words

CORRECT:
class
Pre_email pi.pre_email.php
class Pre_xml ext.pre_xml.php
class Pre_import mod.pre_import.php

For example,
Why class name is Pre_email, but the file name is pi.pre_email.php, should it just be pre_email.php? How the prefix pi works here?

Is this section just talking about add-ons style guide? Where are the add-ons information in the user's guide

But in other sections of the style. We are supposed to use lower_uderscore_filename.php as file name, right?