This type of field can keep binary values (i.e. values that contain zero-bytes inside) with the length up to 64K bytes.
This field always uses N bytes on disk per value. It is easy to see that FixedBinary field is similar to fixed-size String Field by storage type.
This field can be declared as Nullable. In this case a special bitmap is associated with this field.
This field type is represented at the API level by VFixedBinary Class.
At SQL Level you can create such field using
CREATE TABLE T1( fldFixedBinary FIXEDBINARY(30) );
You can use this field in any predicate expression, e.g.
SELECT * FROM T1 WHERE fldFixedBinary = 'abcdef';