Use case
Return the number of sub-tasks in the Done or Complete statuses of the Sub-task issue type.
Expression
Create and configure a Smart field using this expression in General parsing mode:
%{count(filterByIssueType(filterByStatus(subtasks(), "Done, Completed"), "Sub-task"))}
Please, note that by changing "Done, Completed" into other statuses and "Sub-task" into other issue types, you can adapt this use case for your needs.
