Optional sessionA client session identifier string.
If specified, the app will start sending ViewStateMessage updates to the sender of this ping-pong message, and those updates will be tagged with this session ID. This is useful if a client is communicating with multiple apps and its messaging transport mechanism prevents it from identifying the origin of the various messages that it receives.
Arbitrary text that will be included in message responses.
The tag identifying this message type.
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A "ping" or "pong" message.
If you send this message to the app, it will reply with its own PingPongMessage that repeats your threadId and sessionId.
If you're trying to communicate with the WWT research app through the postMessage() web API, there's no surefire way to know that the app is actually receiving your messages. This is a particular issue when the app is starting up. The least-bad way to account for this is to periodically send pings and wait until you start getting replies. The threadId parameter makes it possible to distinguish messages between multiple instances of the app and multiple app clients.
This message also includes a sessionId field that allows clients to set up customized tagging in ViewStateMessage updates, which is helpful when a client is messaging with multiple apps and needs to distinguish their replies.