If something is customizable, you can change it so it fits a particular person, job, or situation — the settings, colours, size, extra parts. Software, phones, cars, and even school courses get called customizable. It is a selling word in shops and tech: fully customizable means they want you to feel it can become yours. American spelling is customizable; British spelling is often customisable. The verb is customize / customise. Though actually, it is not the same as customary, which means what people usually do. And custom as tradition is a different path. The custom that lives in this word is custom as in made to order.