I have bumped into this concept using Python
I did google it, but didn't fully grasp the idea,
so would rather get a better explanation from someone more experienced to better assimilate the concept.
"Trove classifiers are for classification (and searching is a nice consequence of that). It's a matter of accuracy. PyPy, IronPython and Jython are not programming languages they are implementations of the Python programming language. Shedskin and and Cython are python-like programming languages (yes, subset and mostly-superset respectively)."
"It would be useful to be able to specify that a package is tested with (known to work with) alternative implementations. So, for example, I would tag "mock" with pypy and Jython as I routinely test on those implementations and know that it works."
This was a new concept for me, thus please complete this answer or start a new one so it may help others who run into this in the future,
It's actually a project,
"The Trove project is an attempt to create an open-source distributed archiving system for use at large software archive sites."
And it's specified in PEP 301
"The list of classification values on the module index has been merged from FreshMeat and SourceForge (with their permission). This list will be made available both through the web interface and through the register command's
--list-classifiers option as a text list which may then be copied to the
setup.py file. The register command's
--verify option will check classifiers values against the server's list."