Jira applications let you add custom fields in addition to the built-in fields.
Custom fields are always optional fields. This means that you can create a new custom field without requiring existing issues to be changed. The existing issues will contain no value for the new custom field.
Adding a field directly to an issue
Jira Admins can add an existing field or create a custom field while in View Issue with the Admin > Add field option. You can even configure the options for that custom field without having to leave the screens you are presented with.
Adding a field using the Add Custom Field button
Choose :cog_icon: > Issues.
Select Fields > Custom Fields.
Select Add Custom Field. Select Advanced.
Select "Hierarchical Calculated Field" and click Next.
Configure the selection criteria "Name" and "Description" for your field, as shown below:
The Field Name will appear as the custom field's title in both entering and retrieving information on issues, whereas the Field Description is displayed beneath the data entry field when entering new issues and editing existing issues, but not when browsing issues.
6. Select Create.
7. Choose which options to display and select Submit to finish adding your new custom field.