It's an alternative to the standard N75 'F' or 'C', depends what model you have.
It controls the opening/closing of the wastegate.
The J is generally seen as being more agresive. I ran 0.2bar more with a J, compared to a C.
The J gave more low down + mid range power, but lost out slightly topend to the C.
You can buy these at any VAG dealer, and are roughly £55 inc VAT.