With a "Lookup", you can use a multi-column referential constraint, external CSV files or data from an external database as a data source. You can also specify a custom query and process a certain item for each row in the result set of that query.
Lookups are defined per item and can be used as a source for data for that particular item only.
For more information on Lookups, see the Lookups chapter.
The sample project makes use of Lookups.
A lookup as used in the sample project.