Snap-In-Ones (SIO) are a type of all-in-one or complete nappy. The only difference is that the insert is not sewn in, but can be buttoned in and out again with snaps. This way, if the insert gets just a little wet, you don't have to wash the whole nappy right away. The nappies come with one insert, additional inserts are available separately. If the child usually makes a lot of pee at once, it is usually necessary to wash the whole nappy.
Here you can learn in detail how SIO work.