An essential diagnostic tool for AWS VPCs.
Egress-Only internet gateways allow outbound (and response) only access to the public AWS services and Public Internet for IPv6 enabled instances or other VPC based services.
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Egress-only is what you want when you want to mimic the OUTBOUND-only functionality you would get with a NAT Gateway for IPv4 traffic.
Use roughly for the same thing as Gateway endpoints but how they are implemented can be radically different.