Soapbox Mailer's templating system is built to provide an easy interface for communications professionals to quickly put together an email. Here's how the three core parts of the body of an email work together.
Email Template - this provides the core design elements of the HTML version of the email. This typically includes the background, header, footer, etc.
HTML Body - this is the content that is merged into the Email Template, based on where you've inserted the HTML Body merge field in the Email Template.
Text Body - this is the text-only version of your email body, and is not related in any way to the Email Template.