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Tags are a subset of hierarchies + symlinks. If you have a one level hierarchy, you have the equivalent of tags.

I'd argue that tags provide a more graph-like structure, of which a hierarchy (tree)[1] is just a subset. Symlinks provide an "escape-hatch" to hierarchical systems allowing objects to appear to be in more than one directory.

[1] https://en.wikipedia.org/wiki/Tree_(graph_theory)

How so? Unless you can tag tags, you can't represent a tree with them. If you can tag tags, you get an arbitrary graph, with all the problems that represents.

I don't think I've seen tagging systems that let me tag tags out in the wild.

You can create a DAG from Tagged objects if you use Set operations. The set of things with tag A includes things from other sets. Those sets form the next level in the hierarchy.

The tagged objects define the relationships between differing tags which allows you to define that arbitrary graph. The graph can be viewed as an effectively infinite depth tree if you wish to.

The 'path' in the tree can be represented as a tag. It's a simplistic way to go, but you can get pretty far with it to emulate a folder structure using tags.

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