体裁 and 题材 differ by one tone and get mixed up by native writers too. This one is the form a piece takes, novel, essay, poem, play. 题材 is what it is about. A war novel has the 体裁 of a novel and the 题材 of war.
memory
裁 is the tailor's character, to cut cloth to a pattern, the same one in 裁缝. A 体裁 is the pattern a text is cut to.
