Considered by the CAA to be a 'normal part of a flying duty period' I.E. you are
expected to be sufficiently rested to be able to operate into discretion
unless unfit. It
IS 'compulsory' in as much as if no-one declares themselves
'unfit' they have to do it. Try
'rejecting' extension into discretion and your career will be shorter than you reckoned. Yes it is the Captain's responsibility to 'assess'.
How do I know - I wrote and asked, because it became necessary.
