Hi Yaron, in elasticsearch (even in version 1.4) similarity is per-field. So idf part is based on the field "scope", meaning the count of documents which contain this field.
In version 7.x , I think (did not test) you can still use tf/idf , even if deprecated, by setting the index-level similarity to "classic", as in similarity | Elasticsearch Reference [7.11] | Elastic
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