After you have loaded the file definitions (i.e. made the file known to LANSA), you should add the file validation rules and triggers, virtual fields, batch control logic and so on. A brief description of some of these facilities is given in 5.4 Review the File Definitions.
At this stage, you should also review the field definitions and add descriptions, validations, help text and so on. How to do this is described in Building the LANSA Repository - Fields.
Whatever options you set for this file and its fields, the details will be maintained purely by LANSA and have no impact on any "external" applications that use the file.