Line items are sometimes used for the sole purpose of creating filtered views. For example, I may have a list hierarchy that contains two level: Employees rolling up into Region. And for one particular view, I only want to show a flat list of Employees, regardless of the Regions they roll up to. To do this, I could add a boolean line item, set the formula as "TRUE" and then make sure that summary method in the Blueprint is equal to "None." Then in the filter, I would show list items where the boolean was checked. Since the summary method in Blueprint is equal to "None," parent list items (i.e. Regions) will not be checked and will automatically be filtered out. Note that it is possible to filter based on line items in other modules.