Depending on the complexity of a system, a URS issued to prospective vendors may well be subject to considerable revision before the final set of requirements is agreed.  The system design, which by this stage may comprise a number of individual documents, such as drawings, specifications, data sheets and a plethora of clarification communications, has to be pulled together to avoid any confusion as to the full extent of the system.

This is the job for a System Definition document, which is a compilation of the information comprising the system design from the variety of source documents.  It not only identifies the physical boundaries of the system, but also provides a complete description of its installation and operation.  What may sound a considerable chore is not much more than a cut and paste exercise, with all the information these days being available electronically.  It certainly pays dividends, as any description of the system required in any downstream document, whether a validation plan, qualification protocol, operating or maintenance procedure, etc. simply cross-refers to the appropriate section of the particular version of the System Definition.