We may ask you to try these so we are sure we don't ship bugs with the next release. These are development versions that are not intended or recommended for live sites and should only be used on staging sites for testing.
Despite having a version number this is not a fixed state but work in progess.
This is the master or main branch in GitHub terms and represents the next official bug fix/maintenance release. This can be a minor 1.6.x or patch 1.x release and usually has version numbers like 1.6.1a, 1.6.1b, 1.6.1RC
The support build should be compatible with the official release but it cannot always be guaranteed especially on a version jump like 1.5.x to 1.6. and if security fixes require specific changes.
There is currently no development build
This is the development built for the next major release. So if 1.6 is the current release, this will become 2.x and will get all the new features we made up. This release may introduce incompatibilites due to serious changes.