One of the subtleties is how to determine acceptance criteria by the stakeholders/client. You can negotiate upfront a spec, and upon delivery conformance to the spec means success. Or you could negotiate a high-level objective, or a metric to be changed, and every sprint get an on-the-fly approval of the incremental changes delivered.

The key is understanding the upfront spec, while a necessary stating point, is almost certainly wrong, maybe fundamentally so. So how are renegotiations handled?