The 3604 is because a variable length file requires a way to identify how long each record is. IBM determined, many many many years ago, to use a 4-byte prefix on each record. The first two bytes of the prefix have the record length and the second two bytes are reserved. Hence if your COBOL program has a maximum length of 3600 for a variable length record, the LRECL will be 3604. The block size must be at least 3608 (there's a 4-byte block length prefix for each block since the block length needs to be kept somewhere, too).