The Metalness options are covered in
https://docs.chaos.com/display/CRMAX/Corona+Converter (similar documentation should follow for C4D I believe). Basically it's have Corona make an educated guess on whether it should be a Metal material, or have it always create a Metal material, or never create a Metal material. Use Auto unless you find your plants and grass are turning into metals, in which case use never which is No Metal (usually there are less metals in a scene and so easier to manually go through and adjust those after conversion). Unfortunately many pre-Physical Material materials were set up with unfeasibly high IORs for instance, which should only happen on metals, so may be interpreted by Corona as metal during conversion.
I haven't tried the second, I'd say that either it converts the selected shaders, or it converts all shaders in select objects, letting you choose whether to select objects as a whole and convert everything on them, or to select specific shaders that you want to convert.