Actually, source and source-code, doesn't make any sense in the site. I've checked out some questions with both tags and saw, pretty much software installation, compilation, make questions. No questions about "source code" itself.
Software Sources is a GUI application to manipulate the sources.list:
GUI-based repository management is normally accomplished via "Software Sources". This interface can be accessed via several methods. For the latest versions of Ubuntu, the easiest way is to go through the "Ubuntu Software Center". Open the software center, then from the Edit menu select "Software Sources".
This is entire different to what "sources" or "source code" wants to mean.
install-from-sources seems like a subset of software installation. Answerers rarely cares if you want to install something from sources or the compiled binary. From sources it may involve extra steps (the infamous
./configure && make) but in essence all questions about installing something from sources are installing software. No need to fragment anymore the topic.