An Initial Product Design Specification (PDS) is a list of everything about your product. Specifications are used to tell the potential buyer about the product. If you return back to the example ISR of the Circuit timer it states things about the user, testing etc.. When you have produced your ISR you will have came to a number of conclusions:

What your product will be used for.
What size it is going to be.
What type of decorative finish will be applied.
How you will ensure safety.
Cost Etc… Etc… Etc…

The specification again may be developed as you work through your project. Changes that may be needed during the project have an effect on the specification, you may want to remove some part of the design and change something else. Make notes on the reason you changed something and record it under design development and ongoing changes.
The Final Product Design Specification will arrive when you have completed all your designing, development and testing and had some user feedback. 

For a more comprehensive list to help with your specification look at Design and Make It Resistant Materials p10 and 11.

As part of your research you could look at commercially produced specifications in catalogues etc...

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