Filtering input correctly for the Zettelkasten (slip box)
Only valuable notes are allowed in the slip box. These are insights, i.e. thoughts independently formulated by oneself. 
Collection of information, references, potentially once useful may not be included in the slip box! Otherwise the slip box will quickly become confusing, full of what is only temporarily valid .
The note box must not become a universal record or general project file. These things belong either in a chronolgically kept journal or in project files.
On the other hand concepts, procedures, or key insights don't belong in the project file, but in an overarching repository (Anki or slip box).
A collection of information becomes more complex and cumbersome the larger it gets. The (well-managed) box of notes is different. It becomes more useful as it contains more ideas .
This positive dynamic arises on the one hand because there are simply more and more useful things in the note box, but above all also through the links that grow superlinearly with the content and thus promote creativity and new insights .
: Sönke Ahrens, Das Zettelkasten-Prinzip: erfolgreich wissenschaftlich Schreiben und Studieren mit effektiven Notizen (Norderstedt: Books on Demand, 2017).