When you choose fields, each one of them have specific properties associated with them.
For key, summary & linked issues - ARN supports 'display as link' property. This allows you to configure whether the Key/Summary is formatted as a hyperlink in your template.
By default, Jira url of the issues are used. But if necessary, one can also use customer portal urls instead. This is helpful when target audience of your release emails are your customers who don't have access to Jira.