I've searched around but couldn't find any documentation on this.
I was happy to discover the autostart application functionality! I would be more happy if there was a way to specify *where* to start the program. There's a line in the .config/autostart/foo.desktop file called Exec=foo. Perhaps there's some syntax like this Exec=foo --workspace3 ?
You can log out and save your session with the apps open on the workspaces and xfce willl start the as you left them.
or you could use devilpie.
I'm aware of the session and devilspie option. However, the developers saw it fit to include an autostart application function - functionality "normally" covered through sessions and devilspi - and I can't help but think there must be some more than this. I hope this will be expanded on in the final release