An addon domain lets one hosting account serve a completely separate website on a different domain name. Visitors see an independent site; behind the scenes it is a directory inside your account, sharing the same disk, bandwidth and PHP resources as everything else you host there.
That sharing is the part worth understanding before you add several. This covers creating one, the three names cPanel asks for, and how addon domains differ from subdomains and parked domains. A distinction that causes more confusion than the setup itself.
Addon, subdomain, parked: which one you want
Addon domain. A different domain, a different website. example.net serving content that has nothing to do with example.com.
Subdomain. A section under your existing domain: blog.example.com. Same domain, separate area. Covered in creating and managing subdomains.
Parked domain, sometimes called an alias. A different domain showing the same site. This is what you want for example.net pointing at your existing example.com content, and for keeping alternate spellings of your brand.
Choosing addon when you meant parked gives you two separate empty sites instead of one site reachable by two names. It is the most common mistake on this screen, and it is easy to undo before you build anything.
Creating an addon domain
In cPanel, open Addon Domains. Type the new domain and cPanel fills in the other two fields automatically.
New Domain is the domain itself, without www.
Subdomain / FTP Username is generated from it. This becomes an FTP login and is also created as a subdomain of your main domain, which surprises people the first time they see it. Leave the suggestion alone unless you have a reason.
Document Root is the directory holding the new site. The suggestion is usually public_html/example.net, which is sensible: each addon domain in its own clearly named folder.
Optionally set an FTP password to create a dedicated account limited to that directory. Useful if someone else maintains that site.
Add the domain. cPanel creates the directory and the DNS entry immediately.
Point the domain at the server
Creating the addon domain in cPanel does not make it load. The domain must also point here, which happens at your registrar rather than in cPanel.
Set its nameservers to the ones in your welcome email. Propagation takes anywhere from a few minutes to a day, depending on the previous time-to-live value.
Until that completes, the addon domain will not load and nothing is wrong. Check the domain's current nameservers before investigating anything else. It is the answer in the large majority of "my addon domain does not work" cases.
The subdomain that comes with it
Every addon domain is also reachable at addonname.yourmaindomain.com. That is how cPanel implements it, and it is genuinely useful: you can build and test the site before the domain's DNS has moved.
It has one side effect worth knowing. That address serves identical content to the real domain, so search engines can index the same site at two addresses. If the site matters for search, set a canonical URL or redirect the subdomain form to the real domain once the site is live.
SSL
Each addon domain needs its own certificate. Ahosting includes free SSL, and it is normally issued automatically once the domain resolves to the server.
The order matters: DNS first, then the certificate. Requesting a certificate before the domain points here fails, because the certificate authority cannot verify you control the domain. If issuance failed, fix DNS and retry rather than retrying alone.
Check the SSL status page after adding any domain. An addon domain without a certificate shows visitors a browser warning, and nothing else will tell you.
What addon domains share
The sites are separate to visitors and not separate underneath. They share your disk quota, your bandwidth, and the PHP processes your plan allows.
That has two practical consequences. One site consuming resources slows the others. A busy addon domain can make your main site feel sluggish. And if the account hits its disk quota, every site on it stops being able to write at once: email fails, uploads fail, database writes are refused.
They also share a security boundary. A compromised addon domain sits in the same account as everything else, so one neglected site puts all of them at risk. If sites need genuine isolation, they need separate accounts rather than addon domains, which is what reseller hosting provides.
Removing one
Removing an addon domain in cPanel detaches the domain and its DNS entry. It does not delete the files, which stay in the document root and keep consuming disk.
Delete the directory separately once you are sure. Check what is in it first, and take a copy if there is any chance the site is wanted later.
Each addon domain is validated independently, so one can be secure while another quietly is not. For checking, see SSL for Subdomains, Addon Domains and Mail.