The system checks, when you open the file, if the description of the record format you are using was changed since your program was compiled to an extent that it cannot process the file.
The system normally notifies your program of a level check. When you use the create or change file command, you can specify that you want level checking. You can also override the level check attribute defined for the file using the LVLCHK parameter on the Override with Database File (OVRDBF) command.