SD stands for speed density. Basically it means you dont have a mass air flow sensor telling your PCM how much and at what temperature air is coming into your engine.
Im not 100% on specifics, because it can vary from car to car, but it bases the instant tune needs off of manifold pressure, throttle position sensor and an IAT sensor??
That's correct but you still have like you said the IAT sensor, so temperature still matters...
In my opinion speed density is way better than mass airflow.....I've seen a lot of problems with the mass airflow setups.
The difference between the two systems is the MAF sensor vs. the MAP sensor...