You would be surprised how useful the User Requirement Specification becomes when trying to work with your upcoming project.
I want to remind you that documentation is truthfully important. Many people often forget to write detailed items in the User Requirement Specification if they even write a URS at all. But from a programming perspective having a URS can be a huge lifesaver for you in the long run. It lays the groundwork for what you require and desire from the machines programming side of things.
For instance you may want to have a special type of alarm go off, but only for a second or two when an emergency stop is pressed, then allow that same alarm to pulsate 3 times after the Emergency stop has been reset. Yes, you could write this down on a sheet of paper or a notebook, or even a word document for safe keeping, but putting this is in the URS has much more value.
When you get together with operators and technicians, you can get their input on your requirements. So, if they want something that is feasible, be sure to add that, in addition if they see it as something that should not be instilled in their line, then leave it out.
At the end of the day the URS offers you two things, a base level road map of what you need to do to program/create our equipment, and a place to start for equipment testing/validation. These are your requirements; they need to be completed before the equipment is ready to be signed off and transferred into full production.