This is simply a practical way of measuring the amount of toxin so that doctors know how much they are injecting into a specific area. A “unit” of each product has a specific number of molecules of that given toxin, so we know how much of an effect it will have when injected. It is better understood if we use something like salt for example. If 100 tiny little grains of salt are considered a unit, it would be much more practical to say that we added 5 units of salt, rather than 500 grains of salt.