In the current situtation you can chose between "parents", "descendants" and "children" in the configuration of the ClassificationFacetCheckBoxList.
If you select children, you can only get the first level of the classsification tree. However if you want to setup a filter based upon another level you would need to introduce a calculated field based upon the level of the intended classification, wich is a kind of content replication and not a very good solution if need to setup multiple filters. However, if you would be able to specify the classification level in the configuration you could limit the number of facets to what is actually intended. (See example screenshot).
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