This read-only Unsigned32 value is the Security Parameter Index, an arbitrary 32-bit tag that, together with the destination address and the AH/ESP protocol, uniquely identifies one active IPSec Security Association. An administrator troubleshooting a VPN tunnel uses the SPI to match packets captured on the wire to the specific SA entry the device is applying. For example, if a peer reports its tunnel is dropping traffic with a given SPI value, the admin looks up that SPI in this table to find the matching local SA.