Re: VirtualBox: stretch-backports vs third-party repository
>> For this reason I often write instead:
>> apt install virtualbox/stretch-backports
> That's nice.
> Why does this work and are packages installed in this way updated when
> running `apt upgrade`?
I found out myself that
apt -t stretch-backports install virtualbox
will install dependencies from stretch-backports and
apt install virtualbox/stretch-backports
will install them from stretch (or stretch/updates or stretch-updates).
Packages installed from stretch-backports will be updated when a new
version is uploaded there.