How to Setup Email Accounts on Add-on Domain in cPanel

To create email accounts for your addon/subdomain accounts, you'll need to use your cPanel account to first create the appropriate addon/subdomains.  Once set up, head to Home > Email > Email Accounts.

There you can create an email account for the appropriate subdomain.  Be sure to choose the appropriate addon/subdomain from the dropdown menu.

What is the difference between email in addon/subdomains and regular domains?

There is no difference between the two, as they all create a unique email address that you can send to.  The difference only comes from what the name looks like after the @ symbol.