Current Transducer Usage
The reference voltage is what the transducer produces on its output when measuring 0A.
So if 250A measured gives +2V out, then what you'll actually see at the output is 4.5V  (2.5 + 2).
Similarly, for -250A you'll see 0.5V (2.5 - 2).  

It appears that you can also drive the reference voltage to a value of your choosing, so you can set the output voltage for 0A to whatever level is convenient for your application.  

Yes, the scaling is linear (at least within the limits of error shown in the datasheet).