Why the key and type are copied over directly?

[NE-SmallTown]

In the doc:

In fact, when a fiber is created from an element, these two fields are copied over directly.

But I check the src code:

var createFiber = function(tag: TypeOfWork, key: null | string): Fiber {
  var fiber: Fiber = {
    // Instance

    tag: tag,

    key: key,

    type: null,

   ....
  }
}

So,key is from the caller which means it can be null or not,just decided by the caller,but type is null.So,I think "these two fields are copied over directly" is not correct.

And BTW,I know what the type is from the src code's description and react-devtools.But I am not sure what the key means.Does it is what we see in the react-devtools just like type?