You will probably find it useful to have some knowledge of the structure of the DP4 databases used by the applications you are supporting. The system development team should normally supply appropriate documentation to you. If they do not, you can use the DP4 Table Planner to print out a create a schematic diagram of the database.
Even more useful will be a list of the tables on the database, together with their internal reference numbers. Knowing what table number. You can quickly create such a list by using the PRINTDB utility, available as Print Data Dictionary on the Database Management option of the DP4 menu, and just selecting the List of Tables option.
For more information on using these programs refer to Printing the Database Structure in the DP4 Developers' manual.