difference between 'sidebar', 'side panel', 'expand sidebar' unclear

[stevesea]

in the View -> Toolsbars -> Customize Toolbar... page, there are buttons you can 'Sidebars', 'Side Panels', and 'Expand Sidebar'

From the names, I'd expect them to all effect the same widget
'expand sidebar' -- this seems to toggle you in or out of collapsed toolbar layout.
'side panels' -- opens up the Zen Web Panels widget
'sidebars' -- opens up a side bar for bookmarks/history/synced tabs.