Selected dimension values are only returned with a valid navigation query.
Because descriptors (selected dimension values) are only returned with a valid navigation query, the Navigation parameter (N) is required for any request that will render navigation selections:
N=dimension-value-id1+dimension-value-id2[+...]
The Navigation parameter is used to indicate the selections made to the MDEX Engine via this set of dimension-value-ids. These selected dimension value IDs are the descriptors of the Navigation query. That is, the descriptors are what describe a navigation query. The descriptors are what a user has already selected.
The only exception to this is the URL query:
N=0
where the descriptors consist of a single ID of zero that does not correspond to any dimension value. Instead a dimension value ID of 0 indicates the absence of any descriptors. It indicates that no dimension values have been selected. When a navigation query is issued with a descriptor of 0, there will be no selected dimension values to render.
Note that the MDEX Engine combines selections from the same dimension into similar dimension objects. This consolidation is why ancestors and descriptors exist, because they were independent selections, but then combined into one dimension object that relates them by the dimension's hierarchy.
Performance is rarely impacted by rendering the selected dimension values, because rendering selected dimension values is merely a product of displaying what has already been computed. Like other features related to navigation, performance of the system as a whole is dependent on the complexity and specifics of the data and the dimension structure itself.
