Extensions can extend the panx-framework with new features, for example the PostManager. PostManager is one of official extension. PostManager includes templates files, css, classes etc. If you are asking, why PostManager is not the part of panx-framework by default, the answer is, that not every user will use it, so if you want to use it, you need install it additionally.
Extensions can be installed using panx-framework:
php panx-worker extension install {EXTENSION}
The extensions are ZIP files, so it will extract the zip files into panx-framework. Be carefully, that means some of your files can be overwritten, you should backup you project before installing extension.
php panx-worker extension
This command above will display all available extensions.
You can work with extension using panx-framework in this way:
php panx-worker extension {NAME} [parameters]
Where {NAME} is the name of extension, e.g. PostManager.
Creating own extension is easy, just write the extension you want, delete all files that are contained in panx-framework by default and run command:
php panx-worker info extension
After this, just ZIP all files and voilà, you have created extension.
To uninstall extension, execute following command
php panx-worker extension uninstall {NAME}
Where {NAME} is the name of extension, e.g. PostManager.
You should backup your project before executing the command.