I’m not a developer, though I have some experience in control automation and programming, so this is as far as I understand:
CTUN.BAlt - Barometric altitude;
CTUN.SAlt - Sonar (Rangefinder) altitude;
CTUN.Alt - Calculated (estimated) altitude, the altitude which controller assumes as current after processing data from the sensors;
CTUN.DAlt - Desired altitude - the altitude which controller “wants” to be, usually, when everything is right,follows CTUN.Alt with a small time lag.
Again, this is my understanding, not a formal description, somebody from developers would probably correct me…