Packet filter sessions can be configured to be shared between the two firewalls, so that in the event of a failover, the new primary already knows about the current sessions and there is less of an interruption. If you are curious, you can configure this in the packet filter service that you are using. "Stateful session failover" option.
The session flush is probably referring to the session being closed out on the primary, so it is flushed from the standby.
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