The first layer is Chinese characters and numbers plus commas, for example: "one," "two," "three,";
The second layer contains Chinese characters and numbers in parentheses, and cannot be followed by "," or ",", for example: "(1)", "(2)", "(3)";
The third layer is Arabic numerals with foot points, for example: "1." "2." "3.";
The fourth level contains Arabic numerals in parentheses, and cannot be followed by "," or ",", for example: "(1)" "(2)" "(3)";
The fifth layer is Arabic numerals with circles, for example: "①" "②" "③" or "1)" "2)" "3)";
The sixth layer is uppercase English letters, for example: "A." "B." "C." or "(A)" "(B)" "(C)";
The seventh layer is lowercase English letters, for example: "a." "b." "c." or "(a)" "(b)" "(c)";
references:
[1] He Jianmin. Attention should be paid to the standard writing of the sequence number in the manuscript [J]. Applied Writing, 2008(10):26-26.
[2] The structural order of the serial number_Chinese Text Library