The Sum (Average) of numeric values in linked issues custom field allows you to define the value range for the number custom field type.
Sum (Average) of numeric values in linked issues is only available as a custom field type. It is not possible to define general values or any other setting options. Use configuration scheme context to define standard values.
Create and configure the custom field
To create and configure the Sum (Average) of numeric values in linked issues custom field, you must be logged in as a user with Jira Administrator permissions.
Go to Settings → Issues → Custom Fields
Create a new custom field of type CFS -Sum (Average) of numeric values in linked issues
Add name and description
Associate the field to the appropriate screens. You must associate a field to a screen before it will be displayed.
Click on 1 Context and click Create, edit or delete contexts on the dialogue.
Click Edit custom field config.
Define Sum or Average, Issue Type, and Numeric field you want to use.