Trunk is where released language pairs go. They should be functional, and useful in some sense of the word, either for assimilation or dissemination, however they may also be (fully functioning) prototypes.
All trunk pairs should have had at least one release.
The pairs in trunk are in the trunk category in https://apertium.github.io/apertium-on-github/source-browser.html. The files for trunk are located at https://github.com/apertium/apertium-trunk.