In DSDM, date and budget are fixed. The scope is flexible — but only if the plan leaves room for it. That is why one simple rule applies in every timebox: at most 60 per cent of the effort is Must.
Must, Should, Could, Won’t
Must is whatever the delivery makes no sense without. Should is important, but there is a workaround. Could is welcome if time remains. Won’t is explicitly not in this timebox — and therefore not forgotten.
Why 60 per cent
Every estimate is a bet. If a Must requirement takes longer, it takes the time of a Could, later that of a Should. The date holds, and the most important things are done. With 90 per cent Must, there is no buffer: either the date or the quality slips.
What this means for clients
At the end of every timebox you get working software with all Must items. Whatever is moved is in the report — visible, not silent. And you decide whether it goes into the next timebox.