elasticsearch-definitive-guide | The Definitive Guide to Elasticsearch | Search Engine library

I started reading the documentation about Elasticsearch, and I read about _type metadata element, in Elasticsearch documentation:

Elasticsearch exposes a feature called types which allows you to logically partition data inside of an index. Documents in different types may have different fields, but it is best if they are highly similar.

So my question is: In which situations the best practice is to split documents into types? Because in the documentation, they wrote that the documents in different _types should have similar fields.