The default is actually "index". Reindex supports blindly bumping the documents version number (default), only changing the document if the source document's version number is higher (version_type=external) and only creating missing documents (op_type=create). This mirrors the index API which supports those three options with the same parameters. The index API requires you to send the version if using version_type=external but reindex handles that for you, piping the version of the matching document through.
The index API also supports version_type=internaland specifying a version which will bump the version of the document you are writing only if the version matches the one you send like traditional optimistic concurrency control. That is what update-by-query uses.
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