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Prefill templates are regular email templates that you can develop in Email This Issue, however you must understand how they work in conjunction with the templates you use to render the final email.
How Prefill templates work
Prefill templates can be composed using HTML, velocity, may contain issue fields, or regular text or anything.
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it is a good practice to add a distinquishable Template Category name for the prefill templates so that they can be easily identified. So far all is easy.
Templates to send emails prepared with Prefill Templates
Contexts have an Email Template attribute. This specifies the template used to send the emails sent manually. If you configure the Context with a prefill template, you should not use the same template to send the email, otherwise all content will be duplicated in the email because the Body field is added to the template via the variable $!mailBody. Or more generically, Email Templates and Prefill templates should not have overlapping content to avoid duplication of content in the email.
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Prefill mechanism is only applicable to emails composed manually. Event Notifications or Workflow Postfunctions should be configured with regular, detailed templates.