The Dashboard is your driver seat. Access to everything.
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The top half of the Sidebar is usually for stuff that is for visitors of the site.
The bottom half of the Sidebar is for stuff that’s for admins of the site.
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Mousing over a menu item will reveal the sub items. When you’re in that menu section, the sub-items will be displayed.
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You may not see as many items. Access to features depends on your user role capabilities.