Adding custom fields to emails
Apart from getting notifications about updates to custom fields in issues, you can have additional custom fields displayed in your emails. You can display a custom field anywhere in the email, like footer, header, and even email subject.
These tutorials are for advanced users, and are out of scope of Atlassian support. You will be adding custom fields by editing the code in the Velocity templates, which email notifications are based on.
Email notifications inform you about changes in the issue's fields (both built-in and custom fields). If a field's value did not change, it won't be included in the notification, because there's nothing to be notified about. After all, notifications are all about showing a change.
Here you can learn how to include a custom field, and its current value, in every notification for an issue, even if this field wasn't updated. You can use it to describe the issues you're being notified about more precisely. For example, you can include a custom field that specifies the issue's security level, and then properly categorize or even hide the notifications for this issue.
Which email notifications are you using?
Check which notifications are you using, as both types are based on different templates.
Separate notifications are the default notification type in Jira. In this type, every update to your issue is sent in a separate email notification.
Batched email notifications is an optional feature, and needs to be enabled. In this type, issue updates that occur close together are grouped and sent in a single email.
To check whether these notifications are enabled in your Jira instance, go to> System, and open the Batching email notifications page.
Batched notifications were introduced in Jira 8.0, however you can add custom fields only from Jira 8.1.