rkt tries to stay out of the way for everything that isn't "download, validate, and execute a container". For example, it doesn't know about networks but will hook out to CNI to setup a network namespace. It isn't an init system and will instead rely on a system init system to monitor it. It doesn't do any clustering but can be used by things like Kubernetes.
Whilst I'm more familiar with Docker than rkt it seems that both projects are still changing quite quickly and have a lot of movement in terms of features being added & changed...