The Alt-Svc HTTP header is used to inform the clients that the same resource can be reached from another service or protocol, this is useful if you want inform the HTTP clients that your server supports HTTP/2 for example.
Example: if your server is running on a local IP 127.0.0.1 and is listening on 2 ports: 80 (non encrypted) and 443 (encrypted). You can inform the clients, that HTTP/2 is supported on port 443 using the following HTTP header
Alt-Svc: h2=":443"
When HTTP/2 is enabled, automatically adds this header if the connection is not running on HTTP/2 protocol.
You can enable or disable this feature using the property HTTP2Options.AltSvc.