The simple truth in any supply/demand role is that the characteristics of the supply vary constantly. Specification limits are a function of client tolerance. While control limits set the values that are able to be met by the project itself, specification limits are a more fluid and abstract concept. A client may think that a product is perfect, and the next moment find it completely unacceptable. So specification limits are not a viable method of quality control that should be used by project management to evaluate the propriety of a given set of data. Specification limits should certainly be viewed as a guideline, but never as a concrete requirement. The specification limits that are deemed acceptable by the client should always be viewed as secondary to the control limits that actually define the capabilities of the project. The project management team must clearly identify the specification limits, and help the client understand the difference between what is necessary and what is simply preferred. Specification limits are an arbitrary means of measuring satisfaction and, unless clearly defined, should be deemed by the project management to be a non-critical factor during the process.
This term is defined in the 3rd and the 4th edition of the PMBOK.