separate vhds out of rds, used when there are large number of virtual hosts in a route config
HTTP routing with massive route tables
runtime layer discovery
https://docs.google.com/document/d/1-IFziAJYaxQdcbXGyenF6gvVFB-20aPTm5XkwlmcNac/edit#heading=h.xw1gqgyqs5b
allows extension configurations (e.g. HTTP filter configuration) to be served independently from the listener.