What is the difference between a 'System Plugin' and a 'User Installed Plugin'?
System plugins are those that shipped with the product when you downloaded it from Atlassian. These plugins are integral to the functioning of the system, and although you can disable some of them, you should not do so unless instructed by an Atlassian Support engineer. Note, not every system plugin can be disabled and you will not be able to uninstall any system plugins at all.
User-installed plugins are those which have been installed in the product after it was set up: either by uploading a plugin jar file, or by placing it in the applications plugin directories. These plugins can be uninstalled.