Assigning a Cloudfront Distribution ( Account A) an alternate domain name which is a subdomain of an already registered Domain in another Account (Account B)

Yes, it is possible to create a CloudFront distribution in Account A and associate an alternate domain name that is a subdomain of a domain registered in Account B on Route 53.

This process involves setting up a CloudFront distribution in Account A and linking it with an SSL certificate for a subdomain in the same account. Then, a CNAME record is created in the Route 53 hosted zone of a domain registered in Account B, pointing the subdomain to the CloudFront distribution's domain name. This allows accessing the CloudFront distribution at the specified subdomain URL, with no further setup needed in Account B.

An alternative is to purchase and configure your own domain and SSL certificate for the CloudFront distribution in Account A. But using a subdomain of an existing domain registered in another account is supported as described above.