If you give record-format information, it must be compatible with the actual structure of the data set. For example, if you create a data set with FB-format records, with a record size of 600 bytes and a block size of 3600 bytes, you can access the records as if they are U-format with a maximum block size of 3600 bytes. If you specify a block size of 3500 bytes, your data is truncated.