Ontology is the knowledge of relations between subjects.

2022-03-23 on matrix

I once published a review of most common categorization systems in Russian:

I found 7 different types of such systems

In short, translated:

#tags like this, one might call them hashtags
Classic wiki categories
Forward indices
Map of content
I didn't find any more categorization systems. Everything is a mixture between the seven

There's no distinction between linking to the topic and inclusion in the topic with the link+backlink combo