ES-XW7T-1A278-AB is a Ford engineering specification, and the designator repays reading closely because it encodes both the subject and the state of the document. The leading letters identify the document as an engineering specification, the alphanumeric block identifies the subject area and the individual document within it, and the trailing two letters give the revision level, so that the same core number appears with successive suffixes as the requirement is amended. A supplier meeting a specification of this kind should establish which suffix the programme is working to; the difference between two suffixes is sometimes editorial, but it may also be a change of level or of an applicable port, and a test report issued against an earlier suffix does not automatically satisfy the later one. Because the specification is issued through the manufacturer's engineering system rather than through a public standards body, the copy in a supplier's possession is a controlled document, and it should be checked against the customer's system before it is used as the basis of a test plan.