Prototypes made on PoP are based on sketches made on paper or a whiteboard, which are later photographed or scanned and uploaded into the app. Once that's done, hotspots can then be added, which register tapping or several gestures.
PoP is built to be a tool for quick prototyping, especially in participatory design sessions. For complex prototypes with a lot of screens it would require a large number of images to be used and this can make any project hard to maintain.