List & Label also works database-independently in this mode. This means that the application is (i.e. you as a programmer are) responsible for supplying the data. You tell List & Label, by calling a function, which data (fields) are available in your application (e.g. "A Field called <Name>, a field called <Lastname> etc.") and which content this field should have. Where you get the data from at print time is totally irrelevant for List & Label. In most cases you probably perform a read access to a record field in a database.
To integrate the Designer into your application, you need to tell List & Label about the available data fields by calling a function for each of your data fields that may be placed in the form. With this call, you may also optionally declare the field type (e.g. text, numeric, boolean, date etc.) to List & Label, which is e.g. relevant for the correct treatment of your fields and variables in formulas within the Designer. You may pass a sample content for each field, which is used during the design in the layout preview. If you want to support the real data preview in the Designer, please follow the instructions in chapter Direct Print and Export From the Designer.
During printing, the passing of data works analogously, apart from the fact that you have to supply the real data content instead of the sample content. This has to be done for all fields used, while you are iterating all records you want to print out.