What is a Domain Alias in G Suite ?
As the term “Alias” suggests, Domain Aliases are the domain name which will not be used to create new separate emails ids, rather it will link to the email ids you already have with your primary domain.
As an example lets say in your G Suite account you have primary domain named example.com, but as you have a multinational business, you have country specific domains too, like example.co.in, example.fr, example.tk etc. Now if you create 3 separate email ids for each of your employee, say jhon@ example.com, jhon@ example.co & jhon@ example.us. That person (Jhon) has to maintain all three email accounts daily and it would become a nightmare.
Now if you add example.com as primary domain of your Google Apps account but example.co.in, example.tk as domain aliases, then even if someone sends an email to jhon@ example.us it will still go to jhon@ example.com and the person has to maintain only one email id.