By Default it selects a balanced route that includes both paved and good-quality unpaved surfaces. If you want something more “off-roady,” choose the gravel/off-road option in the Settings menu.

You can (on the website) control whether the route between two points is routed as gravel or standard (aka “any”). Plan the route as gravel to start with. Then click a via point and you can set it in the top row there. That way you can mix and match across a route. 

Thanks for your help guys - much appreciated