If you extend electromagnetism to broader contexts, then you will often find that there is now a restriction on the charges of particles. Paul Dirac said could we have magnetic monopoles like a North Pole without a South Pole? He came up with a way to do it and he showed that if there's even just one monopole in the universe,. Then the restrictions of quantum mechanics imply that all the electrical charges have to be quantized. In other words, have to be multiple integer multiples of some basic unit. We have never seen a magnetic monopole, so that may or may not be relevant to nature, but there are certain theories that predict them.