This item is an integer identifier which, along with
_array_data.array_id, should uniquely identify the
particular block of array data.
If _array_data.binary_id is not explicitly given,
it defaults to 1.
The value of _array_data.binary_id distinguishes
among multiple sets of data with the same array
structure.
If the MIME header of the data array specifies a
value for X-Binary-ID, the value of _array_data.binary_id
should be equal to the value given for X-Binary-ID.