Sending Messages to Specific Client Instances - TotallyInformation/node-red-contrib-uibuilder GitHub Wiki

Each client instance (e.g. a browser tab) gets a unique socketId when the page loads. This is reset every time the page is reloaded.

That ID is added to the msg when the client sends to the server as msg._socketId.

The ID can also be added to a msg sent from the server to the client. If that is done, the msg is forwarded only to that specific client instance.

If no msg._socketId is included when sending from Node-RED, the msg is broadcast to every client instance connected to that URL.