The answer to your question is quite simple:
Imagine that you want to group the items in a given data group. So in this case you will have to insert one group descriptor to create the initial groups, and after that another group descriptor to create the sub-groups within each initial group. A simple real-life example would be a list of products that can be ordered in groups of categories and sub-categories after that.
The same is with filtering and sorting.
I hope this answers your question.
Let me know if I can assist you further.
the Telerik team
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