What is the usage of "Additional JQL Function" in the filter?

Use this feature to perform an issue dependent search.

The field "Additional JQL Function" will suggest a list of all custom fields from type numeric, text, single select,… and Jira system fields. Selecting an option from this field will build up the JQL function currentIssueField() with a specified field and paste it into the JQL box. The function performs a calculation on the specific field and returns a value from the issue in which the Issue Panel is displaying.

The full document on Smart Issue Panels for Jira

For example:

• "Epic Link" = currentIssueField("Epic Link_[epiclink]") → returns all issues that have an epic link with the current issue.

• assignee= currentIssueField("Reporter_[reporter]") → returns all issues that have been reported by the reporter of the current issue.

• Client = currentIssueField("Customer_[customfield_10032]")→ returns all issues that have the same value in the Client field as the current issue in the Customer field.

Note that JQL searches do not return empty fields in the results.