Ultimately some things are hard boundaries, but those things tend to be very old and conserved in the genome and if they break then you tend to not become an organism; there is a reason like 2/3 of all successful fertilization's end in miscarriage. Some bits gotta work and cant be messed with, but assuming an organism is brought to term, but for other things there is a ton of wiggle room for variation and mutation. Environment also plays a huge role, like you can have a "malfunctioning" gene that works just fine if you have a healthy environment with little stress. Additionally all these variations functionalities are contingent upon a fractal pattern of other biological and environmental factors.