What is the difference between ingress network and custom overlay network?

thank you very much for the link. After reading it, I have two different understandings,

1 only joining ingress network, we can use routing mesh, if nodes join a user-defined overlay network, routing mesh is not available

2 routing mesh is available on both ingress network and user-defined overlay network. user-defined overlay network . user-defined overlay network uses ingress network as a underlying-proxy to divert traffic to support routing mesh

which one is correct?