DuckDNS Port Forwarding question

I recently changed my routers so I needed to add my port forwarding rules.

443 → 8123
80 → 80
8123 → 8123

I understand that the 443 → 8123 is to access my home assistant, port 80 → 80 is for the Certbot to renew my SSL certificate, but what does 8123 → 8123 do?

Makes https://<HA domain> work the same as https://<HA domain>:8123? Are you asking if HA for some reason requires it be accessible at <external URL>:8123 instead of <external url>? If so, not to my knowledge