A serial file is a type of computer file in which the data has been written to the file in sequence (like a phone directory). Usually such files have to be read from the beginning to find a particular record.

Serial file organization stores records in a linear format, while sequential file organization stores records in a specific order defined by a key field. In serial file organization, records don't have to be retrieved in a specific order, whereas in sequential file organization, records are retrieved based on the key field's order.

Serial file organization provides indexed categorization of data, which many users find helpful. Another benefit of serial file organization is easy access to files.

records in a serial file are not in any particular sequence. so this type of organisation would not be used for updating the master file, and there will be no way to read a particular record unless you read all the records in the file
SERIAL FILE is a collection of records that is organized one after the other with no sequence

the difference is that

sequential organization: this are records are stored and accessed in a particular order sorted using a key field

serial organisation: records in a file are stored one after another. You need to go first file in order to reach the required file.