What exactly is the relationship between NFC and NDEF? Does any NFC device always have to have data in NDEF format? Or is it possible to have NFC but not NDEF (although I think vice versa is not possible is it?) ? What would an empty NFC card be like? Would it still have the NDEF header information but no payload? Or is it possible for the tag to be completely empty, that is, not have any header information either? In that case, would this still be detectable by an android phone or a card reader? What exactly makes a tag to be detected by a reader?