Take off/Land "at your discretion"?
As far as I understand it, an aerodrome controller would issue a take off/landing clearance with the caveat "at your discretion" if the operating surface and/or the aircraft in question is not visible (or only partially visible in the case of, say, a runway). So far so good, but where is that set down in writing? I've trawled through several docs without success. On top of that, isn't every take off/ landing clearance ultimately at the pilot's discretion no matter the conditions?