Master Files and Access Files are stored separately, apart from the associated data source. Your application uses a data source Master File (and if required, the corresponding Access File) to interpret the data source in the following way:
If the Master File is already in memory, your application uses the memory image and then proceeds to locate and read the data source.
If the Master File is not in memory, the application locates the Master File on a storage device and loads it into memory, replacing any existing Master File in memory.
If your Master File references other data sources as cross-referenced segments, or if a JOIN command is in effect for this file, the cross-referenced Master Files are also read into memory.
The data source contents are interpreted based on the information in the Master File and, if applicable, the Access File.