Generally, the index symbol consists of lead and a circle, and the figure number "-"at the lower part of the circle indicates the figure number. The number at the top of the circle indicates the drawing number; In addition, the text on the leader indicates the referenced atlas number.

In a relational database, an index is an independent physical storage structure that sorts the values of one or more columns in a database table. It is a collection of values in one or more columns in a table and a corresponding list of logical pointers to data pages that physically identify these values in the table. The function of the index is equivalent to the catalogue of books, and you can quickly find what you need according to the page numbers in the catalogue.