Build biceps and back with the machine taking the edge off.
Heavier assist means more help, reduce it as you get stronger to earn a full chin-up.
The assisted chin-up gives you the underhand pulling pattern with the machine shouldering part of your bodyweight, so you can train full range reps from dead hang to chest at the bar before you are strong enough to do them solo. The lats drive the pull while the biceps get heavily involved thanks to the underhand grip. Dial the assistance down over the weeks and you are literally watching your progress on the stack.
Enough to get clean, full range reps with a couple left in the tank, and no more. Remember it is backwards from normal machines: a heavier stack means more help, so progress means the number going down.
Chin-ups use the underhand grip, which brings more biceps in and is usually a touch easier, making it the friendlier place to start. Train both across the week if you can, they feed each other.
Usually the assistance never goes down, or reps stay short. Use full range every rep, drop the assist weight a notch whenever the top of your range feels easy, and mix in slow negatives from the bar to bridge the last gap.
