RxCocoa traits that were previously categorized as units are structures that wrap an Observable sequence, and they exhibit the following additional characteristics that do not apply to RxSwift traits. They deliver events on the main thread via the main scheduler, and they are guaranteed not to fail and emit error events. These characteristics make these traits particularly useful for working with UIElements, for example, displaying the most recent value in a UILabel certain to be on the main thread and ensuring that there will be no errors.