Event type Error does not exists

When exploring the specs of the events.Event resource, the field ‘type’ is documented with:

type is the type of this event (Normal, Warning), new types could be added in the future. It is machine-readable.

Does this mean that one should not use other values than the ones defined, so machines can handle correctly values for this field?

Specifixally, there is no trace of an Error type. Is there some justification for this? If so, is there some documentation that makes reference for this decision?