what does the "u" mean in "jdku" in java development kit?

Question

I am seeing it in some download files e.g. OpenJDK21U-jdk_x64_linux_hotspot_2023-08-09-06-56.tar.gz?

Some of the github brances are also suffixed with "U".

Answer 1

It means "update"

Based on the release guide for OpenJDK:

When the repository has been created, a few changes to the codebase will be necessary where the code references a jdk version but not it's new update version. I.e. jdk11 became jdk11u when it was moved to an update repository.