The meaning of the first three optional parameters is exactly the same as in Allow command.
If an URL matches given rules, indexer will download only a little part of the document and try to find MP3 tags in it. On success, indexer will parse MP3 tags, otherwise it will download whole document and parse it as usual.
Notes: This command works only with the servers which support HTTP/1.1 protocol. It uses "Range: bytes" header to download MP3 tag.