The practice of taking reference notes comes from Niklas Luhmann, who avoided writing in his books, preferring to write on index cards.
Anytime a passage spoke to something he was working on or reminded him of an idea previously recorded in his zettelkasten; he would write the page number of where the passage could be found, followed by a brief description of what was said or how it related to his thinking on the back of the index card, he recorded bibliographic information, citing the passages that caught his attention Luhmann does not appear to give himself rules as how many is too many. these were not extensive summaries, just enough information to bring the idea back to his memory