1. In Unicast mode, all packets are received by pivot and then forwarded to a specific cluster member for processing. This way pivot member is responsible for perfect stickiness. No other members are receiving packets related to a particular connection, except for a case when the designated member fails in the middle of connection. FW kernel tables are synchronised through the cluster members, but do not require acknowledgement to proceed with packet forwarding.
2. In multicast mode, all cluster members receive the packet, but only the designated member processes it. The other members just drop it. To ensure this behaviour, cluster performs something called "Flash and ACK", where packets are not forwarded before delta sync is done is confirmed through the cluster. Flash and ACK is causing some performance drawbacks. In some specific cases, such as Threat Prevention, where connection should be streamed and go through deeper inspection, there is a decision function providing perfect stickiness.
In all cases, FW kernel tables as synced.
Case of forwarding is not going through sync and in most cases is not related to Load Sharing at all.