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When using [F] (or [G]), an [L] is implied - that is, the response is returned immediately, and no further rules are evaluated.
... once the rules have been processed, the rewritten request is handed back to the URL parsing engine to do what it may with it. It is possible that as the rewritten request is handled, the .htaccess file or <Directory> section may be encountered again, and thus the ruleset may be run again from the start. Most commonly this will happen if one of the rules causes a redirect - either internal or external - causing the request process to start over.
Deny from doesn't seem to invoke ErrorDocument 403 xxx.php