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Document-level ranking factors
A document-level factor is a numeric value computed by the ranking engine for every matched document with regards to the current query. (So it differs from a plain document attribute in that the attribute do not depend on the full text query, while factors might.) Those factors can be used anywhere in the ranking expression. Currently implemented document-level factors are:
bm25(integer), a document-level BM25 estimate (computed without keyword occurrence filtering).max_lcs(integer), a query-level maximum possible value that the sum(lcs*user_weight) expression can ever take. This can be useful for weight boost scaling. For instance, MATCHANY ranker formula uses this to guarantee that a full phrase match in any field ranks higher than any combination of partial matches in all fields.field_mask(integer), a document-level 32-bit mask of matched fields.query_word_count(integer), the number of unique keywords in a query, adjusted for a number of excluded keywords. For instance, both(one one one one)and(one !two)queries should assign a value of 1 to this factor, because there is just one unique non-excluded keyword.doc_word_count(integer), the number of unique keywords matched in the entire document.
