package management - Can a ppa repository be added to /etc/apt/source.list?

By sudo add-apt-repository '', a repository is added to the /etc/apt/source.list file.

By sudo add-apt-repository ppa:/, I saw that all the ppa repositories are added to /etc/apt/source.list.d dir:

$ ls /etc/apt/sources.list.d/
ferramroberto-sopcast-precise.list
ferramroberto-sopcast-precise.list.save
google-talkplugin.list
google-talkplugin.list.save
kalakris-okular-precise.list
kalakris-okular-precise.list.save
linrunner-thinkpad-extras-precise.list
linrunner-thinkpad-extras-precise.list.save
precise-partner.list
precise-partner.list.save
staticfloat-julia-deps-precise.list
staticfloat-juliareleases-precise.list
staticfloat-juliareleases-precise.list.save
telepathy-ppa-precise.list
telepathy-ppa-precise.list.save
ubuntu-wine-ppa-precise.list
ubuntu-wine-ppa-precise.list.save
venerix-blug-precise.list
venerix-blug-precise.list.save

1. Can a ppa repositories be added to the end of /etc/apt/source.list file, instead?




2. Why are ppa repositories are treated differently from non-ppa ones?




3. Are there other non-ppa repositories treated similarly as ppa ones?




4. Will sudo add-apt-repository '' add a ppa repository to /etc/apt/source.list, or to some files under /etc/apt/source.list.d?