For XBRL and inline XBRL documents the handler is called for each taxonomy reference the document’s chain holds while SkipExternalResources is true, except references inside the document. It can skip a reference, which makes the load fail as a denied reference does, but it cannot allow a reference that is denied, and Data has no effect. While SkipExternalResources is false it is not called. See IExternalResourceHandler for the rules.
The handler is copied with the options and kept for every document the operation loads from them. It is not serialized with the options.