distinctEstimate

Estimated distinct scalar values at this path.

Scalar only. An object or an array contributes to observations and to types but not to the distinct count: hashing whole subtrees would cost the subtree on every document, and "how many distinct shapes does this object have" is not the question an index recommendation asks. Nulls are excluded too — nullObservations already reports them, and folding them in would make a column of nothing but nulls look like it had a value worth indexing.