There are currently (5) existential flags for WOF documents that map to the "standard places response" (SPR) interface. The SPR is meant to be a minimum common response for all services that provide a public interface for accessing WOF documents.
The SPR itself remains to be fully and properly documented.
Existential flags can be used to filter records on ingest. For example the go-whosonfirst-pip-v2 package allows you to filter records to index by one or more existential flags:
https://github.com/whosonfirst/go-whosonfirst-pip-v2/blob/master/app/indexer.go#L98-L175