It is deliberately defined a bit vaguely because what is appropriate will vary from contract to contract : it could be by work type, by subcontract, by cost classification e.g. staff costs or, as you allude to, by time. Regardless, it as the Contractor chooses.
You are correct in that once it has been accepted or assessed, it cannot be revised, except via the disputes mechanism.
You are correct in that once it has been accepted or assessed, it cannot be revised, except via the disputes mechanism.