When the file is opened, the system checks for changes to the database file definition. You can specify the check for record format description changes with the LVLCHK parameter.
You can override the system and ignore the level check using the Override with Database File (OVRDBF) command.
For example, assume that you compiled your program two months ago and, at that time, the file was defined as having three fields in each record. Last week another programmer decided to add a new field to the record format, so that now each record have four fields. The system notifies your program, when it tries to open the file, that a significant change occurred to the definition of the file since the last time the program was compiled. This notification is known as a record format level check.