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In the event that the subscribing client becomes disconnected from the server, the server will publish a LWT message on the same topic setting the state to offline. Any connecting client that subscribes on the state topic will then be notified of critical application state and can key off this to do different things when that subscriber status changes. 

STATE message formats

Prior to the v3.0.0 release of the specification, the STATE message included a UTF-8 string payload that was either ONLINE or OFFLINE. Sparkplug™ B payloads are not used for encoding in this payload and this allowed critical/backend application(s) to work across Sparkplug™ payload types.

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