Assemble offers a union of properties available in any visible model, called tolerant grouping. This feature allows a user to choose any parameter of any visible model as a grouping parameter.
Understandably, not all Revit categories share the same parameters. For example, "structural material" is a parameter of structural categories and not a parameter for mechanical equipment.
This results in the need for additional grouping labels that Assemble designates as "Not Assigned" and "Not Applicable".
Not Assigned is a grouping label for instances that contain the parameter name but have a null value for that parameter.
Not Applicable is a grouping label for instances that do not contain the parameter name (such as mechanical equipment not containing a parameter called 'structural material').
An attempt to load a grouping that contains parameter names which are not in ANY currently visible model, Assemble allows the user to edit those properties in addition to the union of the visible model properties, and indicates that these unavailable properties are not available in any visible model. This allows Assemble users to load any grouping for any view, which can then be deleted or edited.
Also, when editing a grouping, Assemble will show you a warning if a property does not exist in at least one visible model.