What is the difference between a 'System Plugin' and a 'User Installed Plugin'?

• System plugins are those that are shipped with the stand-alone version of the product and included in Atlassian OnDemand. These plugins are integral to the functioning of the system and cannot be disabled.
• User-installed plugins are not included with the stand-alone version of the product and have been bundled with Atlassian OnDemand. These plugins can be disabled or enabled. Some of the modules of these plugins can be disabled or enabled as well, although modules that are integral to the functioning of the plugin cannot be disabled.