Location of standard parameter types code
From Joomla! Documentation
The code for the standard parameter types can be found in the directory
Each parameter type is defined in a separate file which must have the same name as the parameter type. For example, the category type is coded in
This file contains a single class, called JElementCategory which extends JElement. The naming of files and classes used here is not merely convention. The file must have the same name as the parameter type and it must contain a class called JElement[element-name] otherwise the Joomla Framework will not be able to recognise and support the parameter type.