Every document in Elasticsearch has a type. It has long been the recommendation of Elastic to use only one document type per index, but with the release of version 6.0 this has become more than just advice. For new indices, Elasticsearch now only accepts one document type per index, as a first step …

Dec 15th, 2017: [EN][Elasticsearch] Going from multiple types to one type with the Reindex API

Topic		Replies	Views
Reindex multiple types from one index to single type in another index Elasticsearch	4	3612	July 27, 2017
Reindex from 5.5 to 6.2 with different mapping Elasticsearch	4	850	November 14, 2018
How can we reindex with existing documents to a new type along with already existing types using painless Elasticsearch	4	1278	November 23, 2018
Reindex from remote, 1.7.1 to 6.X Elasticsearch	3	866	August 10, 2018
Parent-child document structure Elasticsearch	3	682	January 31, 2019