Stream pane is the main navigation pane. For a file, you can see there the file's main (unnamed) stream, alternate streams if present, and the file properties page.
For logical and physical drives the Stream pane has a different layout: the main and the only stream, several drive-specific views, and a properties page.
Unlike files, drives has only one stream representing the drive contents. Views just display the appropriate parts of the main stream. A view does not have an independent editing history; the one and only undo list belongs to the main stream.