As you might already know, Debian hasn’t included the Virtualbox packages in Debian Stretch. When SolydXK 9 was released we recommended to use the virtualbox.org repository to install and maintain Virtualbox.
To make installing Virtualbox simpler we’ve added the Virtualbox packages from Debian Stretch backports to our own repository. These packages are updated on a daily basis. There is no difference between the Virtualbox packages in the backports repository and our own repository.
If you are currently using the virtualbox.org repository you don’t have to change anything at all. We decided to offer the backports packages in our own repository for your convenience.