The next table crosstabulates a variable from the vehicle level (vehicletype) with a person-level variable (gender). If you scroll back to the diagram that shows the levels structure, you will see that the person and vehicle levels are parallel to each other (on different branches of the tree). This means that the data at the two levels is not directly related to each other. It would therefore make no sense to populate the table at either the person or vehicle level and therefore this is not allowed. However, you can populate the table at a higher level that is an ancestor of both of them. In this example, the only level that is an ancestor of both the person and vehicle levels is the top level. Each cell therefore shows the number of households that have the various types of vehicles and that contain people of the given gender.