Session Owner

In an HA active/active configuration, both firewalls are active simultaneously, which means packets can be distributed between them. Such distribution requires the firewalls to fulfill two functions: session ownership and session setup. Typically, each firewall of the pair performs one of these functions, thereby avoiding race conditions that can occur in asymmetrically routed environments.
You configure the session owner of sessions to be either the firewall that receives the First Packet of a new session from the end host or the firewall that is in active-primary state (the Primary device). If Primary device is configured, but the firewall that receives the first packet is not in active-primary state, the firewall forwards the packet to the peer firewall (the session owner) over the HA3 link.
The session owner performs all Layer 7 processing, such as App-ID, Content-ID, and threat scanning for the session. The session owner also generates all traffic logs for the session.
If the session owner fails, the peer firewall becomes the session owner. The existing sessions fail over to the functioning firewall and no Layer 7 processing is available for those sessions. When a firewall recovers from a failure, by default, all sessions it owned before the failure revert back to that original firewall; Layer 7 processing does not resume.
If you configure session ownership to be Primary device, the session setup defaults to Primary device also.
Palo Alto Networks recommends setting the Session Owner to First Packet and the Session Setup to IP Modulo unless otherwise indicated in a specific use case. Setting the Session Owner to First Packet reduces traffic across the HA3 link and helps distribute the dataplane load across peers.
Setting Session Owner and Session Setup to Primary Device causes the active-primary firewall to perform all traffic processing. You might want to configure this for one of these reasons:

Related Documentation