Slabbing is basically making substacks. Its main purpose is to save you time during the retouching process by reducing the number of source images you have to sort through to get the bit of detail and/or background you want to retouch your target image with. It is time-consuming to have to search through 20, 50, 100 images to find the right one. So making substacks reduces that by a lot. Say you have 50 images and you set the slabbing to make 10 images with a 3-image overlap you’ll end up with around 10 substack images you can then stack into one and also use for source images for retouching. If you’re not using more than say 20 shots to make your stacks, it’s not that critical, but the way Zerene automates it sure does save time in the end.
Also substacks are easier to control in some ways than full stacks with dozens of shots. The alignment gets easier as does the work to control contrast thresholds and Estimation & Smoothing radiuses (for DMap stacks). I’ve only started playing with those variables, but it’s interesting.
Here’s a deeper dive discussion if you’re interested -