A backport commit (often abbreviated to just backport) is a replica of an existing commit applied to a different repository. In most cases the backport commit will be applied to a repository representing an older release, for example creating a backport commit of a commit in the jdk repository for the jdk11u-dev repository.
A backport commit is considered clean if the changes in the original commit are identical to the changes in the backport commit. Note that only the changes has to be identical, not the changed lines.
All backport commits features an additional commit message trailer (a line in the last paragraph of the commit message) -
Backport-of trailer describes which commit the backport commit was created from. The following commit message shows an example: