The paper signature order xn refers to the last xn in the article signature order that studies a certain problem. An article discussing or studying a problem. The signature sequence xn (on letters, documents, etc.) is x( 1), x(2), …, x(n) from small to large, so it is called X( 1), X(2), …, X(n). Therefore, the paper signature order xn refers to the xn order at the bottom of the article that studies a certain problem.