It works almost like "Server" command but takes a regular expression or a string wildcards as it's argument and do not insert any URL into database for indexing. To insert URLs into database use URL command.
"Subsection" is an optional parameter which can take one of the following values: site, world, with world as a default value. If Subsection is set to "site", then a link between two pages is followed only if the pages belong to the same site. If Section is set to world then no site checking is done.
"Method" is an optional parameter which can take one of the following values: Allow, Disallow, CheckOnly, HrefOnly, CheckMP3, CheckMP3Only with Allow as a default value.
"CmpType" is an optional parameter to specify comparison type and can take either String or RegExp value with String as a default value.
Optional "alias" argument allows to provide very complicated URL rewrite more powerful than other aliasing mechanism. Take a look into alias.txt for "alias" argument usage explanation.