If you're confused it's because you are overthinking it. First, don't worry about functional versus non-functional. Just define what you expect from the pen:

1) What color do you want the ink to be?

2) What size tip should the pen have? Fine tip? Broad tip?

3) Should the pen have refillable ink cartridges?

4) How will you refill the ink?

5) What diameter should the pen be to make writing comfortable

6) What length should the pen be for comfort and for storage or ability to carry with you

7) How is the pen made ready to use? Is there an expected cap on the pen? Do you click a button? Do you twist the barrel?

8) What material do you want the body of the pen to be made of? Metal? A specific metal? Plastic?

9) What weight should the pen have? You pen may be deemed higher quality if heavier.

I could probably come up with a few more, but this give you and idea. Now turn the answers into nice requirements.

1) The pen shall have blue ink.

2) The pan shall have a .5mm ball tip

3) The pen shall have refillable ink cartridges

4) The pen barrel shall unscrew to allow for ink cartridges to be replaced.

...You get the idea.

Requirements are about DEFINING the product. So define it! Then you can determine which are functional versus non-functional.