@simon-77 Re:
Just a small point, our licence for testing and stable is FSF Free/Libre & OSI approved:
See: Software update — Rockstor documentation
I.e. “GPL-3.0-or-later AND (MIT AND Apache-2.0 AND GPL-3.0-or-later AND LGPL-3.0-or-later AND ISC)”
The Stable channel is a service associated with a Stable Updates subscription, where we distribute rpms build from the master branch of the same GitHub repos (generally). Where-as testing is build from the testing branch. Sometimes the first Stable release ends up being build from the testing branch, but only at the end of each phase. This updates service is our way to help support the projects development and to attempt to achieve sustainable open source development.
So the license is identical, we just provide a curated update channel for folks less interested in engaging in the testing phase that ultimately leads to each stable rpm release. And we also try to release, at least the first Stable rpm of each phase, into the testing channel before-hand. So the general plan is for the first Stable rpm to be the exact same rpm that was released at the end of each testing phase.
We should really present this all in far fewer words, The Downloads page does make an attempt at this 
.
Re:
Our Downloads page has had a recent table addition which should help on this front.
From that table 4.6.1-0 is indicated as the last published Stable rpm for 15.5. But as already mentioned, we are so near next stable rpm with our current testing phase that it would be fine to use. But only if one is careful not to ‘roll-over’ to what will there-after be the beginning of the next testing phase. One can install from testing, then subscribe to Stable and once it offers a newer rpm than what is installed, that stable rpm will be installed. I.e. the following forum thread details our last, as of writing, testing release that is near, or at, the end of the current testing phase (as of writing):
I’m hoping to squeeze out one last rpm with only a very minor Rock-on cosmetic fix (by @Flox), so keep an eye out for 5.0.16-0. But that is as yet unconfirmed. I also have a little background/dev change that I’l like to make before this next 5.0.16-0 rpm. But again - this is not yet certain.
Hope that helps. The above doc entry, also quoted by @simon-77, explains this in a few more words.