OpenJDK have release cadence of 6 months. but 3/4 of them are Short Term Supported for 6 months only. This package is designed to harbore them. Currently it is build on openJDK 10. LTSs (next is 11) will go as separate packages.
JDK8 is last LTS release of Java platform. It is bringing many cool improvements - http://openjdk.java.net/projects/jdk/8/ and is landing to your RHEL. Where it will be maintained for several years. You will always be allowed to install Used LTSs in build root, and alongside via alternatives.