Below is an example of a relative path directory alias. In most cases you would use Rewrite feature but it is possible to use an alias for a similar functionality. In Relative Path Directory Aliases, Destination is a relative path.
/mail/ -> webmail/
This makes sure that if somebody goes to http://server/mail/... it will redirect him to the IceWarp Web Client directory in the web site repository.
So all files loaded thru the URI /mail/... will be in fact read from /webmail/.
Notice the missing "/" at the beginning of the destination value. This is a mark of relative directory alias.
I believe it's all clear now.