Q:

Does mod_rewrite set `Alias` automatically or should it be explicitly specified when doing a `RewriteBase`?

From: RewriteBase Apache Docs
        #  /abc/def/.htaccess -- per-dir config file for directory /abc/def
        #  Remember: /abc/def is the physical path of /xyz, i.e., the server
        #            has a 'Alias /xyz /abc/def' directive e.g. #

        RewriteEngine On

        #  let the server know that we were reached via /xyz and not
        #  via the physical path prefix /abc/def
        RewriteBase   /xyz

        #  now the rewriting rules
        RewriteRule   ^oldstuff\.html$  newstuff.html 

Does these lines:
        #  Remember: /abc/def is the physical path of /xyz, i.e., the server
        #            has a 'Alias /xyz /abc/def' directive e.g. #

mean that mod_rewrite sets Alias /xyz /abc/def' automatically based on the .htaccess file or show this be specified explicitly in the conf file?

A:

It does not set the Alias automatically: you need to explicitly define it in your server config, not .htaccess

