>After closing a form with a private datasession, I notice that the datasession (now called UNDEFINED!) are still available from the VFP Datasession (View) dialog. If selecting it, the tables are still open. How come that the datasession isn't released when the form closes?

This is usually because one or more cursor opened in that datasession cannot be closed due to some unfriendly condition- Row buffering and update conflict, or uniqueness violation, etc. To clear this up, make sure you either commit or revert changes to cursors in that datasession.