Do you mean tasks and "let obj=createshot..."? I already know how those work but I can't find anything related to layers in the tutorials or wiki. And the tutorials about object bullets are for 0.12 (and don't mention layers either).
Layers are as follows. Please note that this only applies for Render Priorities between 0.2 and 0.8.
Let's say you have a bullet rendered with the default render priority (50?). Now, if we were to do the following:
let obj = CreateShotA1(192, 224, 0, 0, 0, 0);
Obj_SetRenderPriority(obj, 0.51);
Then this new object would render above all other shot objects on the screen due to its render priority being higher than those of the other shots. In fact, it will render above all things with a render priority below 0.51. If multiple objects are spawned with the same render priority, the ones created later will render over the ones created earlier.