When you compute the horizon stack directly from the Geomodel, it is not possible to choose the horizons to stack together. The user has to choose an interval, the number of horizons and the attribute(s) to map.
Nevertheless, the user can choose all the horizons to stack together if the horizon stack is computed from the Sequence Stratigraphy add-on module. In that case, the user can define by hand the limits of the sequences, within which the horizon stacks will be created.
To save or extract a specific horizon from a horizon stack, right click on the top bar of the viewer and select the option called "Create Horizon". By default, PaleoScan will save and extract the current displayed horizon but a top, a bottom and a step can be defined to save and extract a set of single horizons.