A sheets_id is a spreadsheet identifier, i.e. a string. This
is what the Sheets and Drive APIs refer to as spreadsheetId and fileId,
respectively. When you print a sheets_id, we attempt to reveal its
current metadata (via gs4_get()). This can fail for a variety of
reasons (e.g. if you're offline), but the sheets_id is always revealed
and is returned, invisibly.
Any object of class sheets_id will also have the
drive_id class, which is used by googledrive for
the same purpose. This means you can pipe a sheets_id object straight
into googledrive functions for all your Google Drive needs that have
nothing to do with the file being a spreadsheet. Examples: examine or
change file name, path, or permissions, copy the file, or visit it in a web
browser.