In an mail template body (3) and subject (2) you can insert special tags called template variables:
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In case of an email subject, you must type it manually or copy the variable string from the email's content.
They are dependent of an issue (1↑) for which an user wants to render template. Template Variables come from available fields from selected issue type (so e.g. sub-task issue type may not have all of the variables present in a task issue type)
When you insert variable i.e. ${issue.key} into the body's content, then Email Task will replace this value during sending en email from a specific issue. You can always preview your template variables added on subject and email body on your mail template in preview dialog for selected issue.