Ambiguity is the enemy of development.
1Defining the Edge Cases
The main flow is easy. Great ACs define what happens when things go wrong. What if the user enters an invalid email? What if the network is down? ACs should cover these 'Unhappy Paths'.
2Shared Understanding
The process of writing ACs with your team is more important than the document itself. It's during this time that you uncover technical constraints or design inconsistencies.
3Pass/Fail for QA
The ACs are the foundation of the QA testing plan. If a feature passes all ACs, it is 'Done' from the Product perspective, regardless of how many other ideas people have.
