Email Templates
From WHMCS Documentation
The email templates allow you to customise the messages that go out to your customers when actions occur inside WHMCS. Every email that goes out to a customer can be customised here.
To edit an email template, go to Configuration > Email Templates and click the edit icon next to the template you want to change.
You can also toggle plain text vs. HTML email sending on a per email basis from the editor page.
To view a list of the data that you can include in your emails, click the "Merge Fields" button at the bottom of the editing page. The merge fields you can use depends upon what the email is related to so you can't always use the fields from one template in another - only those of the same group.