To generate reports that have been designed by you or your end-users and send them to the printer or display them in the print-preview all data/records to be processed is being passed on to List & Label. Depending on the programming language this will either happen under the surface automatically when List & Label directly accesses your application's data by using specific data providers or it is being done explicitly by your source code, e.g. if your data is not stored in a database at all. A mixture between database data and application specific data is possible, too.
Besides sending the report to the printer or preview, List & Label offers various other output formats such as PDF, XHTML/CSS, XML, RTF, XLS, DOCX, TIFF, JPEG, PNG, plain text, bitmap and others. This is achieved by special export modules. From the developer's point of view there is no difference between printing and exporting the report.