Joomla! 3 is the latest major release of the Joomla! CMS, with 3.7 the eight support release in this series. Please note that going from 3.6 to 3.7 is a one-click upgrade and is NOT a migration. The same is true is for any subsequent versions in the 3 series of the CMS. That being said, please do not upgrade any of your production sites to the release candidate version as release candidate is ONLY intended for testing and there is no upgrade path from a Release Candidate.
What is this release for?
This is a RELEASE CANDIDATE release, that should only be used on test sites. Release Candidate releases cannot be upgraded.
Extension and template developers are encouraged to work with this release in order to prepare extensions for the stable release of Joomla! CMS 3.7, though there shouldn't be any backward compatibility issues. Users are encouraged to test the package for issues and to report them in the Joomla! CMS Issue Tracker. Please do not upgrade any of your production sites to the release candidate version and do not start a new site on release candidate as a release candidate is ONLY intended for testing and there is no upgrade path from Release Candidate.