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 wrinkles:

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

imply that
mod_rewrite sets Alias /xyz /abc/def' instantly in line with the .htaccess file or show this be specified clearly within the conf file?

It doesn't set the Alias instantly: you have to clearly define it inside your server config, not .htaccess