What do you mean by a subset of classics and history?

Four categories of traditional book classification in China. Classics refer to Confucian classics and primary school books. The history department includes all kinds of history books (official history) and some geography books. Subdivision includes works and religions contended by a hundred schools of thought. The collection includes poems, essays, ci, fu, etc. Generally refers to the ancient classics of China. Source: The Book of the New Tang Dynasty: "Each of them has four books, followed by A, B, C and D, which are listed in the four databases of the subset of classics and history."