What is the advantage of having subordinate CAs (Certificate Authorities) set up under the root CA (Certificate Authority)?

Having subordinate Certificate Authorities (CAs) set up under a root CA provides the significant advantage of allowing for different certificate policies based on usage. This hierarchical structure enables organizations to tailor their certificate management to meet specific needs or compliance requirements across various departments or applications.

Subordinate CAs can operate under their own defined policies, which helps in managing the issuance, lifecycle, and revocation of certificates in a more granular manner. For instance, one subordinate CA might be designated to issue certificates for internal software applications, while another could handle certificates for public-facing web services. This flexibility enhances security and operational efficiency, allowing organizations to align their issue and management processes with specific use cases.

In contrast, while subordinate CAs can issue certificates directly and perform validation of digital certificate requests, these functions are part of a broader management framework enabled by their positioning under the root CA. Similarly, while they could theoretically act as root CAs for external domains, this would not be a standard or common practice, as it typically involves a significant level of trust and delegation of authority that may not align with the original purpose of the CA hierarchy.