Count the number of subtasks, which are open (in status category To Do or In Progress).
Configuration
Create a new Calculated Number Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional
Parser expression
Add the following expression:
count(filterByPredicate(subtasks(), ^%{00144} = "To Do" OR ^%{00144} = "In Progress"))
%{00144} is the field code for Status Category
The ^ is used for referencing the field values of the issue list l - in this case, subtasks(). Without, the field codes would refer to the current issue. [Schema for function filterByPredicate(issue list l, boolean expression predicate)]
To achieve this, the following functions are used: