The first two optional parameters specify comparison and susbstitution types. The default type is Prefix and Case (case insensitive prefix match).
Alias can be used to organize search through a master site by indexing its mirror.
It is also useful to index your own site directly from the local file system, to avoid invoking HTTP server into indexing process and thus offload the server.
Note: Direct indexing from the local file system works mostly in case of a static site. If your site has many dynamic documents, such as CGI scripts or PHP programs, indexing from the file system may not work as expected.
If an Alias command is specified for some site only in indexer.conf, URL substitution is done by indexer just before downloading a document, while search.cgi will display document addresses in the master site URL notation in search results.