EndNote and Citavi rely on proprietary systems for their citation styles. While Zotero and Mendeley work with the open and widely used .csl technology, EndNote and Citavi developed their own formatsthat are not compatible with with .csl files. This means that you either have to use the existing styles for these programmes or use proprietary editors to make changes.
The decision to rely on proprietary systems is clearly due to the fact that these providers want to bind their users to them. Instead of relying on a open, jointly developed format like .csl, they want to force their users to their to stick to their ecosystems. This makes switching between programmes or using the same citation styles across several tools unnecessarily complicated.
If you are looking for flexibility and openness, programmes such as Zotero well advised. I always recommend switching to Zotero, because it only has advantages and on top of that is also free.
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