I've run into this a few times when installing apps from source, and during the occasional hack with update-alternatives. So far, it's only been a minor annoyance (ie, not got in the way of the end-goal) but it's now a frustration as it's pointing to a hole in my knowledge-base... so when I get a message that 'foo' is "not a registered application" (or I can't use foo's default icon cuz Ubuntu has no knowledge of 'foo'):

(1) what defines a "registered application"?

(2) how can I define an application installed from source (and likely residing in $HOME/bin/app-name) such that it packs the same functionality as a package installed from a .deb? (if the solution is not self-evident from answer 1)

Example:

I download and unpack daily dev builds of sublime-text-2 to /home/tom/bin/sublime-text-2. I've created a *.desktop file with appropriate shortcuts, etc. But the icon for sublime cannot be display in any launcher even if I provide a full pathname to the option. The solution is to install a 2nd instance of sublime from a deb package.

When I install sublime-text-2 from a .deb package, it installs under /usr/bin && /usr/lib, the installed .desktop file is stored under /usr/share/applications, and the relevant line reads: icon=sublime_text.

Where's the linkage I'm missing? Somehow Ubuntu knows how to exact the icon from sublime_text in the latter, but not in the former (again, even with a full path provided).