Kotlin doesn't have methods with void return (as Java or C do). Instead, a method (or, to be precise, an expression) could have a Unit type.

A Unit type means that the expression is called for its side effects, rather than its return. The classic example of a Unit expression is println(), a method invoked just for its side effects.

Unit, like any other Kotlin type, extends from Any and could be nullable. Unit? looks strange and unnecessary, but is needed to keep consistency with the type system. Having a consistent type system have several advantages, including better compilation times and tooling:

anyMachine.process(Unit)