Fixed means that one physical block on disk is one logical record and all the blocks and records are the same size. This format is seldom used.
FB (Fixed Blocked)
This format designation means that several logical records are combined into one physical block. This format can provide efficient space utilization and operation. This format is commonly used for fixed-length records.
F indicates that the records are fixed length...
FB indicates that the records are fixed length as well as blocked.
Say for example, we have a file of LRECL 80 bytes and Blocksize is 8000.
That means 1 Block consists of 100 record. In real time when we are reading record from file in cobol program, it is called as I/O processing(Input/Output). TO avoid the no of I/O processing time, we are using the file that defined with FB.