In an active/passive HA setup, the recommended process for upgrading involves minimizing downtime and ensuring traffic continuity by using the failover process:
Suspend the active firewall: This triggers a failover to the passive unit, making it the active unit.
Upgrade the former passive (now active) unit: With traffic now running on the previously passive unit, upgrade the suspended unit while the active unit continues handling traffic.
Confirm proper operation: Once the upgrade is complete, verify that the upgraded unit is functioning properly.
Fail traffic back: Once the upgraded firewall is confirmed to be working, fail the traffic back to the original active unit and upgrade the remaining firewall.

When defining a new VSYS, PAN-OS allows administrators to set explicit resource quotas on policy-related objects, including limits on rule capacities, which can include SSL decryption rules as part of security policy resources, enabling controlled allocation of configuration and processing capacity per VSYS.

The HA3 interface, a Layer 2 link using MAC-in-MAC encapsulation, enables packet forwarding between active/active firewalls to handle asymmetric routing and ensure proper session setup when traffic arrives at the non-owner peer.

External zones act as logical representations of another VSYS and are not bound to any physical or logical interface, enabling inter-VSYS traffic flow, and they are security objects that belong to a single VSYS, allowing security policy enforcement between VSYS contexts.

NetFlow is a Layer 3 (network layer) protocol that collects and monitors IP traffic flows. It is typically configured on Layer 3 interfaces because it relies on IP information for traffic flow analysis, which is not available on Layer 2 interfaces. Layer 2 interfaces handle frames within the local network, and they don't have IP-related details that NetFlow uses to generate traffic statistics.