Normally the service type is used to allow you to connect to peers that are running the same application, so it my be "my-chat-app" for example.
When a peer starts browsing for others to connect to it hears any advertising peer for the specified service type, which generally means those running the same app.
When a peer starts advertising its interest in joining a session it may provide discoveryInfo - a dictionary of key-value pairs that are made available to browsers.
The browser can use the discovery info to decide whether to issue an invitation. The chat room in question could be included in this info.