It’s both and neither.
The thing is, tags and their usage guidance are not first-class citizens among the elements of the site. They’re subservient to the task they have, which is to usefully collect together questions on a given topic. Topic drives tagging.
What topics get asked about is emergent, and tags follow that descriptively.
Not all tags that emerge are accurately or usefully reflecting a topic, so our tags are sometimes winnowed, adjusted, or defined based on a prescription for what would be most useful.
How people use tags, descriptively, is the raw material input to the tagging system. What we eventually observe about which are useful ends up with discussions about how we should use the tags best, which becomes prescriptive.
This is also iterative. When a tag is well established, and has a prescribed usage and topic, it can still evince some rough edges. Sometimes this indicates that there’s something wrong, and discussion observes how it’s actually being used rather than how usage was prescribed, and perhaps a new prescription is in order.
In all this, the topic a tag serves is primary.
For an example, we used to have magic-item-creation. A prescribed usage was easy and established early, but that wasn’t enough to make it a well-behaved tag. It was getting used inconsistently, and also getting used redundantly with both crafting and magic-items. We decided that it, despite descriptively covering a topic and emerging naturally, and having a trivial prescriptive use, was not actually needed. We settled on prescribing usage of the pair magic-items crafting in its stead, rather than honour the emergence of a dedicated tag. It accurately and precisely served a topic, but wasn’t being used well, and we didn’t need it, so it was eliminated.
So the principle for tagging is: a tag should be usefully representing a topic, easy to naturally choose by the uninitiated, easy to use correctly for the initiated, and not cause problems. Tags are a mix of descriptive and prescriptive usage, whichever best serves, at a given stage of a tag’s life, to make it practically useful to the community and future readers.
And ironically, all that is just a description itself. Ultimately, tags serve at the community’s pleasure, by whatever process we decide to use.