A mailbox is an actual email account that can send, receive, and store email. Examples include addresses such as you@yourdomain.com or billing@yourdomain.com.
A mailbox is different from an alias. An alias forwards or routes mail, but it is not the same thing as a full account with its own login and storage. When deciding what to create, think about whether the address needs a real inbox or only forwarding behavior.
In most business environments, shared roles such as support, sales, or billing may use either a real mailbox or an alias depending on how your team wants to handle incoming mail.