Extras are most commonly stored in 2 directories when they are installed:
- core/components/ - This is the location for all the PHP and non-web-accessible files for the Extra.
- assets/components/ - This is the location for the web-accessible files for the Extra, such as CSS, JS and images.
Why the separation? Well, since MODX users can move the core outside the webroot, separating out the non-accessible files into core/components allows MODX developers to add an extra level of security to their Extras.