A shopkeeper might keep a record of each transaction made in the shop in time order (a file) over a 24-hour period. This file will hold details of each sale; what was sold and how many of each item. This file (also known as a ' file') can then be used at the end of the 24-hour period to update the file of products. The master file is a record of the products and what is in stock. It doesn't change except when it is using the transaction file. Once updated, the stock control system can then be used to re-order items. It is also worth noting that updating the master file using the transaction file is done using processing. In batch processing systems, all the data is collected together in one place over a period of time (in the transaction file) before being processed. Once a transaction file has been used to update the master file, the transaction file can be out. It will then be ready for a new transaction period. Master files are . Transaction files exist only for the transaction period, and then they are cleared. It is also worth noting here that while a serial file can be used for this application, a more efficient way to process the transaction file would be to re-organise it into a file first.