A Multi-Domain SSL certificate, also known as a SAN (Subject Alternative Name) SSL certificate, allows you to secure multiple domain names and subdomains with a single SSL/TLS certificate.

This makes certificate management easier by allowing multiple domains to be secured, installed, and renewed under one certificate.

For example, a single Multi-Domain SSL certificate can secure:

• example.com
• www.example.com
• example.net
• shop.example.com

Each domain name included in the certificate is added as a separate SAN.

Important Billing Change from September 2026

Starting September 2026, Certum will change how domains are counted.

Under the new billing rules:

Every domain name included in the certificate will count as one billable domain.

This includes domain names with a www prefix.

Previously, the www version of a domain was treated as a free domain. From September 2026, it will count as an additional billable domain.

New Domain Counting Rules

Starting September 2026, every domain name included in the certificate will be counted.

Using the same example:

Domain: certum.pl
Billing Count: 1

Domain: www.certum.pl
Billing Count: 1

Total Billable Count: 2

Therefore, adding both the base domain and its www version will consume two billable domain slots.