All the open windows are grouped into the respective Apps. Other Open Windows is a placeholder for the open windows that are not a part of any App. For example, consider that you perform a search in "Thing Domain Search" App by loading a saved search. This open window is a part of Search Thing Domain app. Further, if you click on any Supplier in the search results, the Entity Manage page is displayed. As the Entity Manage page is not a part of any App, this is grouped in Other Open Windows.
