Many thanks for your fast answers.
A VDI is a flat single file on the Host,...
I assume, that means the VM / virtual box is one single file, so each VM consists of just one single file on the host disk / the real existing system.
About your other questions: Read the Forum FAQ and the QuickClick FAQ, its all in there.
Sorry, I couldn't find the information there, must have missed anything, but thanks to Sasquatch I do know now.
So if I see right, I assume, this wouldn't work: to install e.g. Office 2007 / Outlook (or any other application) in a box / on a VM and having all personal data (or the data of any other application) like e-mails, addresses, settings, files like .pst on my primary system / the host. And vice versa, installing Office on the real system and storing all the data on a VM. And so I couldn't back up files, folders being on a VM to my real system and vice versa. And this kind of data exchange doesn't work also from one VM to another one, I assume.
Many thanks again, nice greetings, Dirk