The Version sub-menu allows you to manage document versions based on whether they are checked in or out, or if they are checked out by you or another user. The sub-menu is disabled for TIFF and PDF documents.
Check In: Enabled if the highlighted document has a lock icon next to it and it is checked out to you.
Undo Checkout: Enabled if the highlighted document has a lock icon next to it and it is checked out to you.
Versions...: Enabled for all non-TIFF and non-PDF documents and you have the appropriate permissions for version control.