When the data source includes transactional data (multi-record case), the transactions must be aggregated to the case level in nested columns. In transactional data, the information for each case is contained in multiple rows. An example is sales data in a star schema when mining at the product level. Sales is stored in many rows for a single product (the case) since the product is sold in many stores to many customers over a period of time.
See Also:
"Using Nested Data" for information about converting transactional data to nested columns