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Hello J.,
1/ For:
a diagram showing the relations between classes but hiding all the attributes/methods
You can use:
hide members
Example:
2/ And for:
another diagram showing all the detail of attributes/methods but no relations: i.e. just a list of big boxes.
A possibility is to use new style functionality:
<style> classDiagram { arrow { LineColor transparent } } </style>
See also doc. here:
If that can help,Regards,Th.