Table schema:
CREATE TABLE Documents (template_id VARCHAR); CREATE TABLE Templates (template_type_code VARCHAR, template_id VARCHAR)

Question: 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