Sub-templates are children of your master HTML template which allow you to create multiple branded templates all from the same code base. This means you can create templates for multiple brands and products without any HTML knowledge and any updates or fixes to the HTML will apply to all sub-templates.
For each template, you can set design defaults such as logos, colours, CTA styles etc. or content defaults such as footer text or the navigation. This means when you're creating your emails all of this will be set automatically, saving you from doing it manually each time.
Below is a diagram of two sub-templates created from one master HTML template.
Creating a sub-template