A Single-Domain certifiacte only works for exactly one domain. For example, a single domain certifiacte www.mycompany.com will only work for exactly this address (including all subpages e.g. www.mycompany.com/products). However this kind of certificate will not be working for a different subdomain e.g. products.mycompany.com - For this domain you will have to purchase a separate SSL certificate.
Alternative: For supporting multiple subdomains you can also simply purchase a wildcard certificate. - A Wildcard certificate will work on ALL subdomains. If you purchase a wildcard certificate for e.g. *.mycompany.com, it will be working for www.mycompany.com as well as forproducts.mycompany.com as well as for any other possible subdomain, e.g. news.mycompany.com, etc.
