The JHtml class is a utility class responsible for rendering HTML elements.
JHtml::$includePaths is an sequence of path directories that JHtml::_ traverses to locate the relevant JHtml sub-classes to import and call;
JHtml::_($key [, ...])
The JHtml::_ function is responsible for loading an abstract JHtml sub-class and executing a public static function within that sub-class, passing an arbitrary number of parameters passed in the call to JHtml::_ to the relevant function of the sub-class.
JHtml::_ uses JPath::find in conjunction with the array of include paths provided by JHtml::$includepaths to locate JHtml sub-classes.
JHtml::addIncludePath($path = '')
JHtml::addIncludePath adds a path, or an array of paths to the head of the array of include paths provided by JHtml::$includepaths.