According to some Elysee officials quoted by the Guardian
All weekend, France took the initiative with Germany, Ireland, Donald Tusk’s team and a few other countries, to fix the terms of the extension very precisely: that the withdrawal agreement isn’t renegotiable, that the UK would follow a code of conduct and allow the EU’s 27 members to meet to discuss other issues for their future [such as the budget], and that the UK must legally appoint a commissioner if the European commission sits before the UK leaves.
Why has France insisted on this latter issue, i.e. the commissioner nomination?
Previous news on this matter suggested that this wasn't much of an issue in practice:
Sir Tim Barrow, head of the UK’s delegation to the EU, said in a letter to the Commission and Council that his government will not put forward a candidate’s name by today’s informal 26 August deadline.
In his Friday letter, Sir Tim cites Prime Minister Boris Johnson’s 25 July pledge not to pick someone to serve in Ursula von der Leyen’s incoming administration, which is set to take office on 1 November.
But what remains less clear is what would happen if the UK requests and is granted an extension to the 31 October deadline, given that the new Commission is likely to be largely on the starting grid by the time the next Council summit is held on 17 October.
Current UK representative Sir Julian King, a politically unaffiliated diplomat tasked with the security union portfolio, could yet be reappointed to be a place-holder during an extension period, although the tone of Friday’s letter makes it less likely the decision can be rolled back.
So I'm struggling to understand the emphasis of the French request. Is the EU totally unable to reappoint King without an explicit UK request?