There is a default release payload built in to the installer (which can be rewritten by oc when extracting the installer from a payload in a mirror), which the user can also override in the install-config. However, when generating manifests from an install-config, the original install-config apparently gets deleted (in order to prevent the asset model overwriting changes to the manifests). (Note that this doesn't seem entirely consistent with the problem described in AGENT-136, so this description may be missing some detail.)

So all of the data in the install-config needs to be represented at the manifest level (in our case, this means the ZTP manifests). There is no current representation of the release payload mirror in the ZTP manifests (you can only pick a version and the payload location is supplied to the assisted-service in its config), so we will need to add one.