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4) In here we can distinguish:
Issue type (2) - Mail Email template tied with a particular issue type, will be available for selection in issues with this issue type (it'll be visible only with such an issue type, not with other issue types - i.e. in the above image mail email template "Email Template No.1 for Task Issue Type" is available only for issue type "Task"). It determines template variables which you can use in Subject and Content of your email.
Please refer to this documentation table of contents and check "Create Mail Email Template" or "Edit Mail Email Template" if you want to create a new one or amend an existing one.
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- Recipients (1) - You can specify three type of recipients: To, Cc and Bcc. Each recipient can be one of the following type: Email, User, Group and Field. Fields comes from available fields in Issue. As the name indicates, they are to be present as default recipients, when you select particular mail email template tied with an issue type. Example of default recipients management is presented on the below image:
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For more information and more details about those elements please refer to Create Mail Email Template [2.4.x]