Domain Aliasing and Secondary Domain
Adding domain alias and secondary domain are two different things that serve the organization in two different ways in order to manage their domains.

For using a corporate Email solution the first thing we need a domain on which the email ids will run. A Mail Exchange (MX) record is a verified record that specifies the email server responsible for sending and receiving Emails for that domain.
Now, it can happen like the organization owns two or more domains. If you have a G Suite Account you might have heard the word Domain Aliasing and Secondary Domain.
What is domain Aliasing?
Domain aliasing in G Suite refers to giving an alternate name to the domain by which it can be recognized. It is like having multiple front doors to one location. It allows you to host a Website on one domain and point other domain names to the site.
For example, if your domain name is test.com, you can add a domain alias xyz.com and referring any of the domain would point to one address.
Secondary Domain:
The secondary domain is just a separate or additional domain with its own users. An organization adds a secondary domain when it has two separate domains meant for two different business purposes.
How to add Domain Alias or Secondary Domain in G Suite Account?
In order to add a domain alias follow the steps:-
Go to the domain section, from the admin console part of your G Suite account.
Click on Add/Remove domains, and select the option to add domain alias or Add another domain.
3. Enter the domain alias and click on continue and verify domain ownership.
Features of adding Domain Alias :
When you add a domain alias :
Each existing user gets an Email id at both the primary and the domain alias.
The group Email Ids also gets an Email address at the domain alias.
Upto 20 domain aliases can be created and the additional cost is there for any user or Group Ids.
Every user can send and receive Email from either ids and the messages arrive in the user’s current inbox.
Features of adding Secondary domain :
When you add another domain / secondary domain :
You can create users for another domain.
Each domain will have a different set of users thus making it easier to manage a separate team of users for a different business.
Each user will have their own G Suite account, Email address and the payment is in accordance with the number of email ids in the additional domain.