The NPOESS program uses Unified Modeling Language (UML) to describe the format of the HDF5 files produced. For each unique type of data product, the HDF5 storage organization and the means to retrieve the data is the same. This provides a consistent data retrieval interface for manual and automated users of the data, without which would require custom development and cumbersome maintenance. The data formats are described using UML to provide a profile of HDF5 files.