Thanks @mcorr.
Just to add: “Custom” in this documentation is the Red Hat or Foreman definition which differentiates between Red Hat (and SLES) which creates its products and repositories with the corresponding integration automatically and Upstream repositories. For most users “custom” would mean their very own packages where you can skip some steps like Sync plan, Upstream URL, and Download Policy as there is no repository for it and you only upload your own RPMs.
I add adjusting this for the documentation to my ToDo list, but do not expect me to find some time very soon. 