One of the key elements of any application design is the definition of a set of standard methods to identify how different pieces of the application interact with each other, and what functionality each piece is responsible for performing.
The recommended methodology is based on the a three-tier architecture, where we identify 3 layers between the user and the data:
Each layer runs on a different system or in a different process space on the same system than the other layer.
In a enterprise data are distribuited in different Data Stores: mainframe repository of data (e.g. VSAM, ADABAS), Excel Spreadsheets, Active Directories, etc.
A Business application will gather information from these different data stores based on the application of specific business rules
Once all the data is available it will be presented to the user in a specific format, depending on the user preferred interface.
The recommended approach identifies a set of guidelines to facilitate the integration among these different layers.