This INTEGER selects what the packet filter does to matching traffic on this virtual interface: permit, deny, hand off to the firewall, or apply one of the SPF (stateful packet filtering) variants including hardware-accelerated and reverse modes. An administrator sets this to control whether matched packets are simply forwarded, dropped outright, or routed into deeper stateful inspection, which directly shapes what traffic reaches or leaves the box. For example, an admin securing VI 4 facing the Frame Relay uplink might set this filter's action to spfHW so return traffic for an established session is hardware-accelerated instead of re-inspected packet by packet.