System is everything that is not an application: accounts, groups and permissions, search, widgets, menus, notifications, the messenger, themes and the privacy tools. It cannot be switched off, because there would be nothing left.
The applications are Forums, Downloads, Pages, Blog, Events, Gallery, the Knowledge Base and Clubs. Each is a separate module that can be turned off in Admin → Modules.
🚨 They genuinely are independent. Every one of them depends on System alone and none depends on another, so a site that runs Convoro purely as a photo gallery is a supported arrangement rather than a broken forum.
Switching one off hides its pages, drops its menu links and stops its widgets being offered. Nothing is deleted.