When you create a Lookup transformation using a flat file as a lookup source, select a flat file definition in the repository or import the source when you create the transformation. When you import a flat file lookup source, the Designer invokes the Flat File Wizard.
Use the following options with flat file lookups:
Use indirect files as lookup sources by configuring a file list as the lookup file name.
Use sorted input for the lookup.
Sort null data high or low.
Use case-sensitive string comparison with flat file lookups.