minDistinct

Distinct values a path needs to be worth an inverted index.

Two, because an index over a path with one value returns every document and has cost nothing but space. The upper end is maxDistinctFraction, and it is unbounded by default for the reason stated there.

It does not apply to IndexKind.SHREDDED_COLUMN, which is not answering an equality question: a column exists so that a scan of one field never touches the documents, and it pays in proportion to the bytes it avoids reading whatever the cardinality is.