Short: A9 refers to Amazon’s search/ads tech; the actual keywording technique here is TF‑IDF, which surfaces terms frequent in a document but rare in a corpus.
Theory: keyword candidates are terms that are common in your input but uncommon in a broader reference. Comparing to a general corpus elevates unusual domain terms; comparing to a related corpus filters those down to what is distinct even within the domain.
View standard stopwords | View long stopwords