Edit
After digging some more through output of
dpkg --get-selections | xargs apt-cache policy | grep -B5 'multiverse' it turns out there exists meta-package ubuntu-restricted-addons. The /var/lib/dpkg/info/ubuntu-restricted-addons.list pointed me to the file /usr/share/doc/ubuntu-restricted-addons/README.Debian
The contents of that file state:
This package was split into ubuntu-restricted-extras and
ubuntu-restricted-addons to allow the desktop CD installer (ubiquity)
to install a subset of the packages available in the original
ubuntu-restricted-extras package.
Moving forward, ubuntu-restricted-extras should only contain
multiverse packages, whereas ubuntu-restricted-addons should contain
main and universe packages, as well as any packages from multiverse
that have been vetted by a lawyer.
Since ubuntu-restricted-extras depends on ubuntu-restricted-addons,
users should be discouraged from installing ubuntu-restricted-addons
outside of the desktop CD installer. This is the reasoning behind the
warning and sparse description in ubuntu-restricted-addons.
-- Evan Dandrea Thu, 05 Aug 2010 16:14:43 -0400
Thus, Ubuntu installer allows you to install ubuntu-restricted-addons meta-package.
apt-cache tells us what goes into that package:
xieerqi:
$ apt-cache depends ubuntu-restricted-addons
ubuntu-restricted-addons
Recommends: gstreamer0.10-plugins-ugly
Recommends: gstreamer1.0-plugins-ugly
Recommends: flashplugin-installer
flashplugin-installer:i386
Recommends: gstreamer0.10-plugins-bad
Recommends: gstreamer1.0-plugins-bad
Recommends: gstreamer1.0-libav
Recommends: gstreamer0.10-fluendo-mp3
Recommends: gstreamer1.0-fluendo-mp3
Recommends: chromium-codecs-ffmpeg-extra
Recommends: oxideqt-codecs-extra
Conflicts: ubuntu-restricted-addons:i386
This is pretty much the same output as on my 15.10 virtual machine.
Thus , my conclusion is that ubuntu-restricted-addons from multiverse repository is where the third-party software comes from.
Original post
In terms of /etc/apt/sources.list file those would be any software that comes from deb http://archive.ubuntu.com/ubuntu $RELEASE multiverse as well as deb http://archive.ubuntu.com/ubuntu $RELEASE restricted repository.
It's also references in the software-properties-gtk program

You can list packages in those repositories with
awk '/^Package/' /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_$(lsb_release -c | awk '{print $2}' )_multiverse_binary-$(dpkg --print-architecture)_Packages
and
awk '/^Package/' /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_$(lsb_release -c | awk '{print $2}' )_restricted_binary-$(dpkg --print-architecture)_Packages