Hi Sema,
You can list the articles of one category and all of its sub-categories, you can add the field "Article Category Id" to the search form of your search type.
After that, you can click on button 3 in order to select option "Nested Exact" for the field "Article Category Id".
Then, click on button "+". You will have to settings "collection" and "Table". Please select for the parameter "Table" the option "#__categories".
If you follow theses steps, you will obtain all articles of one category and all of its sub-categories.
Regards,
Mehdi.