The local source IP address this gateway uses when originating traffic for this specific tunnel. On a multi-homed gateway with several external addresses, this confirms exactly which of the gateway's own IPs is presenting itself to the remote peer for a given tunnel, which matters because a peer's firewall or NAT rules are often keyed to expect one specific source address. Example: a tunnel that suddenly stops establishing after the gateway's external IP allocation changed, with tunnelSourceIpAddr now showing the new address, would explain a partner's firewall still whitelisting only the old address.