A stacked resole leaves a layer of the original sole and adds (stacks) new rubber on top of it. This can only be done if there is at least 1mm of unworn rubber left on the sole.
To maintain the sensitivity of your shoes select thinner rubbers (3mm-3.5mm) for a stacked resole. To add durability use normal (4mm-4.5mm) rubber.
By doing a stacked resole the shoe can be resoled more times over it’s lifetime. In a normal resole the half sole is completely removed along with a thin layer of the midsole and rand. After multiple resoles, enough material has been removed that they can no longer be resoled.