Many thanks BOAC - indeed it is an excellent site which has answered my ISFD question but which, clearly, does not cater for one of my limited intellectual capacity insofar as I can't find an answer to the "EFIS or PND/ND" question. It clearly can't refer to the switchability of displays between PFD and ND etc as it seems to imply PND/ND as an alternative to EFIS - and that can't be right either. I suspect that the question itself is incorrect and will advise my interlocutor to rephrase it. Many thanks for your help