The DefineBinaryData tag is used to save any arbitrary user defined binary data in an SWF movie. The Flash player itself ignores that data. The size of the data is not specifically limited.
The f_data_id is this object identifier. The identifier is the same type as any identifier (like a sprite identifier.) It is used in ActionScripts to reference the data.
The f_reversed area is 32 bits and it must be set to zero in version 9.
The size of the f_data buffer is defined as the size of the tag minus the f_data_id and f_reserved fields. This is where the raw binary data goes.