I would like to seek opinions on whether the new SSL certs that I purchase should use domain.com or www.domain.com. It seems to mean a different thing when it comes to SSL. I am not sure which is the industry practice.
Hello,
Directadmin doesn't limit you here, and directadmin does not differentiate it, if to say about http/https. www.domain.com is always aliased to domain.com by default.
Check how it's indexed and displayed in search results of Google, and other search engines. That might give a hint.
Usually if to buy a cert for domain.com you can find www.domain.com is also included in the cert. But if you buy a cert for www.domain.com it might be invalid for domain.com (without www).
If your SSL issuer does not allow to include www.domain.com for domain.com then you might want to find another SSL provider.
For other more theoretical aspects of the question: you might want to read what Google suggests here: https://www.google.com/search?q=domain+with+or+without+www