What Is the Difference Between an Add-On Domain and a Sub-Domain?

The two terms describe different kinds of domains on your account:

• Sub-domains are extensions of a domain you already own (for example, anything.yourdomain.com). Because you already own the main domain, sub-domains you create from it are free.
• Add-on domains are additional standalone domains you add to your account beyond your main domain. Depending on your plan, extra hosting fees may apply for add-on domains above your plan’s limit.