of

The term value would be indexed under, or null if it would not be indexed at all.

null for a JSON null and for containers, matching what the writer does — a document carrying one of those has the path, which existence reports, but there is no value to look it up by.

The term value is keyed under given options, or null if it is not keyed at all.

null for a JSON null, for a container, and for a value above IndexOptions.maxTermBytes — all three are present and none of them is keyed. The bound is applied here so that the writer, the reader and a query's recheck cannot drift apart about which values a dictionary can spell; a query that skipped it would take a false negative from an index that dropped the value on the way in.