Acceptance criteria should give a binary yes/no response. They need to be ambiguous in order to test them. Without the binary response it is unclear whether the system is complete and behaves as intended (Writing Great Specifications p. 86)
Speci-fication - (The made up word from Writing Great Specifications) - a document that attempts to be the single source of truth, but can't be because no one is willing to keep it up to date - Speci-fication is "an illusion of correction" - it misleads the reader in thinking that it describes the system as it is, when the system is in fact different.
Many teams fall in the trap of thinking that "Specifying" is an activity that falls in the planning phase, when it is actually a process that occurs as requirements evolve with new understanding
