OpenJDK has release cadence of 6 months, but 3/4 of them are Short Term Supported for 6 months only. This package is designed to harbor them. Currently it is build of OpenJDK 15. LTSs will go also as separate packages.
JDK15 is current release of Java platform. It is bringing many cool improvements - https://openjdk.java.net/projects/jdk/15/. Unluckily, this package is STS (short term support) version. Between individual LTS there will be always several STS. Again, please see announcement: http://mail.openjdk.java.net/pipermail/discuss/2017-September/004281.html. So this is rolling release of all STSs to come. Its fate during the release of fresh LTS is yet to be decided.
