Could we make it a bit simpler yet.....SAS is a "rate" controlled response and ATT uses an "attitude reference". (Thinking Sperry systems....)
SAS tries to keep the pitch, roll, and yaw movement rate equal to "zero". The system senses a rate of movement and applies a countering input to return the rate of movement to zero.
ATT tries to retain a particular attitude reference selected by the pilot. The system senses a displacement from the selected attitude and applies input to return the aircraft to the selected attitude.
SAS mode can be used with Force Trim selected "OFF" or "ON"
ATT mode requires Force Trim "ON".
In SAS mode, the Beep Trim control is inoperative, thus the pilot must manually reposition the cyclic for attitude changes.
In ATT mode, the pilot can either use the Beep Trim Switch to adjust the attitude reference, or reset the attitude reference by releasing the Force Trim momentarily and moving the cyclic to obtain the desired attitude, then set the attitude reference by releasing the Force Trim button.
Minor adjustments or temporary adjustments of attitude can be done by manually over-riding the Force Trim system once the new attitude reference is set.
By all means.....let the helicopter do as much of the flying as it can for you.
I flew with the Force Trim "ON" at all times....and used ATT almost continuouslly. I still have a big ol' callus on my right Thumb!