The IWidgetContext API allows for filtering of top documents within the GUI framework, eg containing a specific set of entities (eg click on one of the bars in the graph in the "Significance" widget). This sub-set is called the filtered documents.
The IWidgetContext API lets you select which of the document sets to aggregate over.
The following diagram provides a visual breakdown of the matching/top/filtered documents, and shows which IWidgetContext calls return data from which documents.