Relenv creates re-producable and re-locatable python builds. The builds created with Relenv are re-producable in the sense that all binaries for the builds are built from source. These builds are re-locatable meaning you can move the root directory around on the filesystem; and even different machines of same architecture.
Relenv can be thought of in a similar way to projects like virtualenv and pyenv. The difference between Relenv and these projects is that each relenv contains the entire python interpereter. Relenv environments can be moved around and shared with others.
Relenv has no public API, but you can find documentation for the internals of relenv here.