An acceptance test specification (ATS) is a document that defines the criteria that a product must meet in order to be accepted by the customer. It is typically used in waterfall development projects, as it provides a clear set of requirements that can be used to create the product design. The ATS is usually created by the customer or client, and reviewed by the development team to ensure that it is achievable. Once the product has been developed, it is then tested against the ATS to verify that it meets the required standards. If the product passes all of the tests, it can then be delivered to the customer. If it fails one or more tests, then changes need to be made to the product before it can be accepted.