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.
Every file stream is completely independent and maintains an independent editing history and undo list.
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.