When getting software from the Ubuntu Software Center I often see that it says "Optional add-ons", like for instance here for rkhunter these are the Optional Add-ons it displays:
Are these optional add-ons made by the developers of the software or at least approved by them? Or are these third-party add-ons? Or does it vary?
I am running Ubuntu 14.10.
If I were to make a guess, the "Optional Add-ons" are what would be called Recommends or Suggests dependencies. They are listed by the package maintainer of whichever package you're trying to install, but may or may not be developed by them or be approved by/known to the upstream developers.
And indeed it is so:
$ apt-cache depends rkhunter | grep -Ei 'recommends|suggests'
|Suggests: bsd-mailx
|Suggests: mailutils
|Suggests: heirloom-mailx
Suggests: <mailx>
Suggests: tripwire
Suggests: libdigest-whirlpool-perl
Suggests: liburi-perl
Suggests: libwww-perl
Suggests: powermgmt-base
|Recommends: <default-mta>
Recommends: <mail-transport-agent>
|Recommends: wget
|Recommends: curl
|Recommends: links
|Recommends: elinks
Recommends: lynx
Recommends: iproute
|Recommends: unhide.rb
Recommends: unhide
Recommends: lsof
