What is special about the hicolor icon theme?

Question

As far as I can recall, whenever I install software using apt-get, I see Processing triggers for hicolor-icon-theme ... (sixth line from top) appear in my terminal window when the installtion is just about complete. An example is below:

Selecting previously unselected package chromium-browser.
Unpacking chromium-browser (from .../chromium-browser_25.0.1364.160-0ubuntu3_i386.deb) ...
Selecting previously unselected package chromium-browser-l10n.
Unpacking chromium-browser-l10n (from .../chromium-browser-l10n_25.0.1364.160-0ubuntu3_all.deb) ...
Processing triggers for man-db ...
Processing triggers for hicolor-icon-theme ...
Processing triggers for desktop-file-utils ...
Setting up chromium-codecs-ffmpeg (25.0.1364.160-0ubuntu3) ...
Setting up chromium-browser (25.0.1364.160-0ubuntu3) ...
Setting up chromium-browser-l10n (25.0.1364.160-0ubuntu3) ...
[08:00 AM] ~ $ 

Hence my question. Is it because the hicolor icon theme is the "ultimate" fallback in case a theme of the user's choice is lacking some icons? In other words, is it because it is an essential "inherit" and complete in all respects?

(I know that chromium-browser is currently a possible security risk because it is quite outdated but I want it just for local use as a web app with svg-editor.html.)

Solution

Googgling around I found this:

In order to have a place for third party applications to install their icons there should always exist a theme called "hicolor"

• This name is chosen for backwards compatibility with the old KDE default theme.

The data for the hicolor theme is available for download at: http://www.freedesktop.org/software/icon-theme/. Implementations are required to look in the "hicolor" theme if an icon was not found in the current theme.

More at Icon Theme Specification (Gnome)/Directory Layout.

And from the package description:

So, yeah it's the fallback icon theme.