Staging is where language pairs go that have been extensively developed, but have not yet reached release readiness. They might be missing small portions of code, or some parts might need to be tweaked. They may also have quite advanced development, but low coverage. It should take around 1—4 weeks work to bring the pair to trunk level (release readiness).
These pairs should build and have an advanced status of all modules (dictionaries with closed categories and decent coverage, an "ad hoc" PoS tagset and .prob (or CG), good coverage of main contrastive phenomena, testvoc clean, and a post-generator if needed). There should be a "PROBLEMS" (or "ISSUES") file saying status and major problems found while reading translations done with this pair (so, not a complete evaluation but a general compilation of big problems detected at the output).
The staging pairs are in the 'staging' column in https://apertium.github.io/apertium-on-github/source-browser.html.