Starting a new custom parameter type

From Joomla! Documentation

To create a custom parameter type you first need to choose a name for it. Suppose you want to call it newparm. Then you will create a new file called newparm.php containing a single class, called JElementNewparm which extends JElement.

The code for the new parameter can exist almost anywhere but it should always be placed in a sub-directory on its own or with the code for other custom parameter types.