Your specifications will change from when you first write them.
They change because:
New information becomes available
Process changes are made
Operational requirements are developed
Because of this change you will need to make sure that you have some version control.
Document management systems can help. Not normally because you want to roll back changes, but because you need to know that everyone are using the latest approved version.
A URS for Process Automation systems is a potentially large document, and having just a single version is inadequate for good version control. A good Functional Requirements model can provide version management down to each object (such as Units, Phases etc) and even provide version control for the data.
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