|Metadata, conditional processing, and indexing / Filtering and flagging|
When filtering a nested ditamap, you should apply the condition attribute to the reference (topicref or mapref element) to the nested ditamap, rather than to the map element.
It is possible in DITA, and often good practice, to have nested ditamaps. Although it does depend on circumstance, it is generally better practice to apply conditions to the topicref or mapref element, rather than the map element of the nested ditamap.
The reason for this, and a point that should always be considered when deciding on a filtering strategy, is the reduced processing load during generation of the output. If the processor has to read in a topic, or a nested map, only to find it's not needed in the current build because of the filtering metadata, it is an inefficient model. It is much better for the exclusion to be notified to the processor before having to load the file.