Fillers are used to bridge the gap between walls and fitted furniture. Often, it's a good idea to make your furniture a little smaller than the room dimensions allow, especially if your walls aren't exactly plumb, or if for some other reason your cabinet design doesn't fit exactly.
This video shows you how to create fillers in Project Mode. By jointly selecting a wall and an adjacent cabinet, Polyboard will automatically drop a filler in the gap. This can be re-positioned and recessed if required. And just like any other panel, you can modify the material and you'll see it in your cut list and plans.