In the Seeheim architectural model, the functional core is the raw functionality of the application, for example databases or API calls invoking buisness logic. The structure of the internal data and logic may not map directly onto the desired user interface, hence the application interface component acts as a wrapper.
Used in glossary entries: API, application interface, Seeheim model
The Seeheim model of the logical components of a UIMS. Figure 8.10 from Human Computer Interaction, Chapter 8, Implementation support