CREATE TABLE Documents (template_id VARCHAR(255)); CREATE TABLE Templates (template_type_code VARCHAR(255), template_id VARCHAR(255))
Which template type code is used by most number of documents?
    
SQL: 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