What field in a domain certificate allows for the use of a single certificate across multiple domains?

The Subject Alternative Name (SAN) field in a domain certificate is designed specifically to allow the use of a single certificate for multiple domains and subdomains. By including various domains in the SAN field, a single SSL certificate can secure not just the main domain but also multiple alternative names, ensuring that all specified domains are protected under one certificate. This flexibility is particularly beneficial for organizations that manage numerous related domains, as it simplifies certificate management and reduces costs.

In contrast, fields like Common Name (CN), Organization (O), and Organizational Unit (OU) are typically used to define single identity attributes and do not provide the capability to address multiple domains under one certificate. The Common Name was traditionally used to specify the primary domain but is now largely supplemented by SAN in modern certificate practices. The Organization and Organizational Unit fields serve to identify the owner of the certificate but do not impact its use across multiple domains.