...

object in the DSpace data model.
For backward-compatibility there is a "DSpace" external registry that
mostly duplicates the format model of versions up to DSpace 1.5.
For more details, see BitstreamFormat + Renovation.

The Data Model

Every Bitstream refers to a BitstreamFormat (BSF) object, which defines its
format technical metadata. The BitstreamFormat, in turn, refers to one or
more entries in external data format registries, which have all of the
actual technical metadata describing the format. A few useful pieces
of metadata are cached in DSpace for each BSF: its descriptive proper name,
its MIME type, etc.

...