Ensuring the use of SSL for data transfer and specifying a KMS Customer Managed Key for server-side encryption via an S3 bucket policy is aligned with AWS best practices. This dual requirement enhances data security by encrypting data in transit and at rest while providing the benefits of controlled and auditable key management through the use of a CMK. The specification of CMKs ensures strict control over encryption keys, enabling tailored permissions and policies reflecting stringent security protocols and regulatory compliance frameworks.