Currently, you can view one subset of documents at a time. Upon selecting a subset, the Concept Cloud will automatically re-display to include only the concepts contained in a particular subset.
Creating new subsets from existing subsets requires simple editing of the source data and reloading the new spreadsheet into a fresh project.
For instance, if you want to compare reviews for people who rated a product both 4 and 5 stars at the same time, you would need to go to your source data set and concatenate a new, unique subset that includes both 4- and 5-star ratings (and upload it as a new project).
Please see "Data Formatting: General Guidelines" for more information.