Dimension collections add detail to a dimension. Each dimension collection is usually configured with a subset of dimension attributes. By filtering the dimension attributes, it is possible to use dimension collections as a method to view items of interest from different perspectives. This can be a powerful tool to ensure deals have the relevant data, ensuring the final deal is focused and succinct.
A dimension collection will then contain a list of dimension collection items with their corresponding dimension collection item attribute values.
Dimension collections are described in more depth here.