By adding an <extends> tag to your package.xml, you tell the world that your new package extends an old package in some way. The new package could supersede the old one, or give new functionality to it.
... <name>Package_Name2</name> <channel>pear.php.net</channel> <extends>Package_Name</extends> <summary>Any one-line summary</summary> ...
Examples for <extends> usage in PEAR
HTML_Common2 extends HTML_Common
HTML_Progress2 extends HTML_Progress
MDB2 extends MDB
MDB2_Schema extends MDB2