After reading Chapters 20, you understand that a physical file is where the data is actually stored. You also understand that physical files may be designated as having or not having a key. A key is one or more fields in the file that are used to establish the primary access path to the physical data. You will probably find that most files you create will have a key, because generally that is how you will access the data.
For example, if you design a Customer Master file, you will probably designate that the Customer ID field is the primary key to the file. This allows you to use the Customer ID field to get to the desired record without having to read the entire file looking for the record in question. The Customer ID field in this case is the primary key to the physical file. But there would probably be occasions when you would want to process the Customer Master file by Customer name. What would you do then? If you are writing a program that will be performed in an interactive environment, your best choice will probably be to use a logical view or logical file to establish your desired access path.