Your channel read marker was updated
{
"type": "channel_marked",
"channel": "C123ABC456",
"ts": "1401383885.000061"
}
The channel_marked event is sent to all open connections for a user when
that user moves the read cursor in a channel.
When an event occurs, we will send an HTTP POST request to your Request URL. The outer payload is uniform across all Events API methods, and contains fields like team_id and event. Learn more