I cannot find any info on what is the difference between the 2 release types - stable and mainstream. I guess stable can be used on production env, what about mainstream? Why would I use one over the other?

>In NGINX nomenclature, “stable” means that no new features are added (the feature set is stable). Only major bugfixes >are committed to that version.
>
>We develop new features and all bugfixes against the “mainline” branch, merging major bugfixes to stable.

https://www.nginx.com/blog/nginx-1-6-1-7-released/