What is special about the hicolor icon theme?

What is special about the hicolor icon theme?

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?
If it is the ultimate fallback, why can't I choose it as my icon theme at least in Lubuntu 13.04? The GUI that comes up by running lxappearance doesn't offer hicolor as a choice under "Icon themes".
(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.)