If you are using the either the AS SIDB Y or the AS 14 SIDB Y, it is because you are running both a side imaging transducer on the transom and a dual beam transducer through the hull for improved high speed readings. Your digital depth reading will come from the dual beam transducer in the hull and your temperature reading will come from your side imaging transducer. 

If you are using a AS SILR Y or the AS 14SILR Y with your Side Imaging unit, you are doing so because you are using two side imaging transducers.  One on the left side of your boat, the other on the right side of your boat.   Temperature and digital depth both come from the transducer mounted on the right side of the boat.