In the context of Kubernetes and Nginx ingress-controller, I can't grasp the difference between an external ingress and an internal ingress.

• what an external ingress and an internal ingress differ in?

• when should they be used and what use cases do they serve?

• when should one use ingressClassName: nginx-internal, ingressClassName: nginx, metadata.annotations: [ kubernetes.io/ingress.class: nginx-external ] aut similis?

I can't find much on the net, that discusses such difference or exemplifies how to use them. There's always some implicit knowledge assumed.