First there's: "the mongod adds the _id field and generates a unique ObjectId to assign as its value." but later in the *Important* note there is this:

"Clock skew between clients can also result in non-strict ordering even for values, because client drivers generate ObjectId values, not the mongod process."

Which is it? Is it the case that the client driver could create an _id field with a local ObjectId, but if it didn't then the mongod creates it?