To my knowledge, there is no automatic setting for SAT speed. When SAT speed is set to 0, it is off. When it is set to 1 or higher, it is on.
On the XLT, all the settings work together to separate the target from the ground. However, SAT and Auto Trac are not directly related.
If you are working in an environment that does not cause the threshold to drift, SAT can be set as low as zero. However, if the environment causes the threshold to drift, you'll have to set the SAT to reset the threshold periodically. The lower the number needed to maintain threshold, the better. If you find yourself having to squeeze the trigger to reset your threshold, you need to increase the SAT speed. If threshold never drifts, you may have the speed set too high. Try dropping it a bit until threshold starts to drift, then move it back up a notch or two. Threshold drift is most noticeable in non-discriminate (all metal or non-motion) mode, but also helps stabilize the discriminate (motion) mode. So, no, it isn't intended only for all metal.
Auto Trac, on the other hand is used to keep the detector balanced. Again, if you are working in a low mineral or low salt environment, you will not need to re-balance your detector very often. If you set the detector to re-balance itself every 8 sweeps or so, you'll be fine. The hazard to having the machine re-balance too often, is that it may balance out a target you are trying to identify or pinpoint.