A footnote by Definition is: An explanatory note inserted at the foot of the page referring to a point within the text, usually indicated by symbols such as asterisks and daggers or by superior numerals. / A brief note at the bottom of a page explaining or expanding upon a point in the text or indicating the source of a quotation or idea attributed by the author to another person. Footnotes are indicated in the text by an Arabic numeral in superscript, or a reference mark, and are usually printed in a smaller size of the font used for the text. When numbered, the sequence usually starts with 1 at the beginning of each chapter but may occasionally start with 1 at the beginning of each page.

Whereas An Index is: # a numerical scale used to compare variables with one another or with some reference number

a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time

list in an index

exponent: a mathematical notation indicating the number of times a quantity is multiplied by itself

an alphabetical listing of names and topics along with page numbers where they are discussed

provide with an index; “index the book” .

At least, when I researched the Diff’s, this is what I’ve came up with…

Hope this helps! ~Ben~