ferred card is located at an entirely different place in the file, frequently in the context of a completely different discussion or subject.
Niklas Luhmann's index card system had three different types of links. Direct links to individual notes, outlines with links to cards (similar to tables of contents or maps of content), and what Schmidt (2018) refers to as "collective references". These collective references sound a lot like search queries for related topics that have links to a variety of resources/cards related to a particular topic and sound like a table of contents, but without a specific hierarchy.