What does the symbol in the red circle mean?
It is clearly meant to represent a current transformer. The data sheet specifies that the transducers can measure current directly or with a current transformer. The advantage of the current transformer is that it provides isolation between the current source and the current transducer.
It is a current transformer. See http://en.wikipedia.org/wiki/Current_transformer
I THINK it's an isolation transformer. The product description does say that it can be used directly or with such a transformer.