Explanation/Reference:
A map tile layer is usually associated with a MapViewer base map, and is managed by the MapViewer server. However, you can configure a map tile layer to cache map image tiles served by an external (non- MapViewer) map provider.
Maps rendered by an external Web map services providers. An external Web map services provider is a server that renders and serves maps upon client requests over the web. If you properly configure an adapter that can fetch maps from the external map services provider, the map tile server can fetch and cache map tiles generated by the external map services provider.
References: https://docs.oracle.com/cd/E14571_01/web.1111/e10145/vis_omaps.htm#JIMPV9783