With respect to Rule 4.16:
4.16 Notification requirements for consumer data requests on joint accounts
(1) If the requester gives, amends or withdraws an authorisation, or if an authorisation expires, the data holder must:
(a) as soon as practicable, notify each relevant account holder through its ordinary means of contacting them; and...
does the clause when referring to an authorisation mean the Data sharing arrangement/consent contract; or
does is it meaning the Joint account election i.e. the indication of disclosure option?
'Authorisation' has the meaning given in the definitions section of the rules (Rule 1.7). So broadly, it's the first of the two options noted in the question.
We have now finalised our joint accounts guidance, which may be of further assistance in considering this part of the rules.