Searching the ProcessWire files for the phrase, "Prevent direct access to file assets owned by" led me to ProcessTemplate.module in wire/modules/Process/ProcessTemplate. From there, this is the relevant code:
/**
* Build the "pagefileSecure" field for the "access" tab
*
* @param Template $template
* @return InputfieldRadios
*
*/
protected function buildEditFormAccessFiles(Template $template) {
/** @var InputfieldRadios $f */
$f = $this->wire()->modules->get('InputfieldRadios');
$f->attr('id+name', 'pagefileSecure');
$f->label = $this->_('Prevent direct access to file assets owned by pages using this template?');
$f->icon = 'download';
$f->description =
$this->_('When direct access to a file in [u]/site/assets/files/[/u] is blocked, ProcessWire can manage delivery of the file, rather than Apache.') . ' ' .
$this->_('This enables the file to be access controlled in the same manner as the page that owns it, while still using the original file URL.') . ' ' .
$this->_('Note that it takes more overhead to deliver a file this way, so only choose the “Yes always” option if you need it.');
$f->notes =
$this->_('Always test that the access control is working how you expect by attempting to access the protected file(s) in your browser.') . ' ' .
$this->_('Do this for when you expect to have access (logged-in) and when you do not (logged-out).');
$f->addOption(0, $this->_('No') . ' ' .
'[span.detail] ' . $this->_('(uses site-wide configuration instead)') . ' [/span]');
$f->addOption(1, $this->_('Yes when page is unpublished, in the trash, or not publicly accessible'));
$f->addOption(2, $this->_('Yes always, regardless of page status or access control'));
$f->val((int) $template->pagefileSecure);
if(!$template->pagefileSecure) $f->collapsed = Inputfield::collapsedYes;
return $f;
}
Considering the attribute name is pagefileSecure, I would think it'd be safe to assume that, yes, that enabled pagefileSecure for those templates within your site. I don't use pagefileSecure myself, so I don't know if there's an adjustment that could be made to the htaccess so that secure pagefiles can work as expected, while insecure can still not be rendered by ProcessWire. There might be some results of people working with that in these forums. (I'd normally look, but unless I revisit this later, my break time is up!)