Templates are dynamically listed based on the contents of the selected language's Templates folder (see below). You may edit the script template for the new script you are about to create by editing the Script Header Contents window (made visible by pressing the "Preview" button). Any changes here will be reflected in the new script to be created, but will not be saved for future use. Any permanent changes or new templates should be modified or added in the language template directory (see below).
To create your own templates, simply copy the desired file to the appropriate Templates folder located in the AddIns folder of the program directory.
Note: also see the ScriptBits™ feature.