Which template type code is used by most number of documents?
SELECT T1.template_type_code FROM Templates AS T1 JOIN Documents AS T2 ON T1.template_id  =  T2.template_id GROUP BY T1.template_type_code ORDER BY count(*) DESC LIMIT 1
SELECT T1.template_type_code FROM Templates AS T1 JOIN Documents AS T2 ON T1.template_id  =  T2.template_id GROUP BY T1.template_type_code ORDER BY count(*) DESC LIMIT 1
[{'Template_Type_Code': 'BK'}]