On May 11 2016 Red Hat has announced the availability of RHEL 6.8. This marks the transition into what in Red Hat's product life cyle is called production phase 2. The kernel level is now kernel-2.6.32-642.el6, for the main bug fixes see the kernel update description.
As usual there are the release notes and the technical notes in two separate documents. The complete documentation can be reached from the Red Hat documentation page (you need to select "6" on the left bar).
The IBM documentation for RHEL 6.4 on developerworks still applies for this release.
From a technical perspective the most important part are the z13 performance patches, which will make certain workloads run faster than on previous releases.