A Mechanic account may be configured with one or more email templates, used by the Email action to render email content using a Liquid that can be shared across tasks. Email templates have access to Liquid template variables named after each option in the Email action, including any custom options added by the task author.
Unless configured otherwise, each email will use the email template named "default", if it exists.
Navigate to your Mechanic settings (it's the "Settings" link in the upper right), and head to the "Email templates" tab. Click the "New email template" link to start editing your new template.
Mechanic will always default to using the template named "default", when present. Feel free to add additional templates – just remember to update your task(s) to use the appropriate template.
Here's an example email action, specifying a non-default template: