I understand thanks.
I set type following your example and I end up in ES with a field type (in _source) and a field _type in the meta data. They both have the same value, the one I used when setting typein logstash.
Should I deduce from this that the field called type in logstash is treated in a special way by the elasticsearch output plugin - by assuming that its value should also be used to populate _type before sending to ES ?