Onion skins are designed to show previous and the Frames after th active frame. This goes up to 10 frames per side. However, it is apparently not possible to exceed this limit. What do you do if you need a frame that lies 100 Frames back? Or more? 10 frames are 1/3 of a second in a 30 FPS movie. Generally the last frame is displayed when your current frame lies after that but how can you display a specific frame like frame 96 and 189 while your current frame is 143 and all layers before and after are already edited?

Never mind, I just found out, that you can define Frames with colors and you can filter these.