Does the Autopilot switch to ALT ACQ mode at a fixed altitude difference from target altitude? For example, if climbing (or descending for that matter) at 3500 fpm, will the AP switch to ALT ACQ mode at the same point it will as when climbing or descending with 1000 fpm? Or does the point of the switch is variable and depends on the ROC or ROD?
I am specifically interested in the B737NG.
The simple answer is that autopilots are designed to fly the aircraft in the same way that a human pilot would.....only more accurately!
If you were climbing/ descending at 3500fpm you’d anticipate the level off earlier than if climbing/descending at 1000fpm or you’d risk exceeding g limits and it wouldn’t be a very smooth transition.
Autopilots, including the 737NG, take into account the ROC/ROD to ensure a smooth transition to level flight so yes ALT ACQ engagement will vary.