In my opinion, taking Permanent Notes is the keystone idea of the Zettelkasten method.
ONE NOTE = ONE IDEA Permanent ideas are supposed to be atomic. In that, they must be fairly independent and generally re-usable in different contexts. The atomicity of an idea is what makes or breaks the usability of Zettels once you reach critical mass.
The trap Block out a few minutes to really introspect how a new note fits into your current repository of ideas. Don’t just collect ideas, collect contradictions, questions and make connections. Run a tight ship, keep your Slip-box from diluting in value.
Tags Don’t worry about broad-strokes Taxonomy. Link each note to a tag that best represents the topic. Suppose you’re learning about Upwind Differencing, which probably isn’t atomic in it’s definition. You maybe wanna link it to something like Differencing instead of say CFD which is much more general. What this allows you to do, is to make a Map of Contents when a topic like differencing reaches Critical Mass.