The `mqtt` binary sensor platform optionally supports an `availability_topic` to receive online and offline messages (birth and LWT messages) from the MQTT device. During normal operation, if the MQTT cover device goes offline (i.e., publishes `payload_not_available` to `availability_topic`), Home Assistant will display the binary sensor as `unavailable`. If these messages are published with the `retain` flag set, the binary sensor will receive an instant update after subscription and Home Assistant will display the correct availability state of the binary sensor when Home Assistant starts up. If the `retain` flag is not set, Home Assistant will display the binary sensor as `unavailable` when Home Assistant starts up. If no `availability_topic` is defined, Home Assistant will consider the MQTT device to be available.
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