What are the layers of data description in R/3?

The external layer: The external layer is the plane at which the user sees and interacts with the data, that is, the data format in the user interface.  This data format is independent of the database system used.

The ABAP/4 layer: The ABAP/4 layer describes the data formats used by the ABAP/4 processor.

The DataBase layer: The database layer describes the data formats used in the database

Conversion between ABAP/4 data types and the database layer is done within the database interface.

Conversion between the external layer and the ABAP/4 layer is done in the SAP dialog manager DYNP.