Ok, helpful follow up here, the film I can only assume is something that may be on the front of the button. The film over the silver disc in the back stays. I just reassembled mine with that film on and home button is perfect. After examining the old home button, I realized that had a film too(just harder to see)

Is there any answer to the plastic film question?? Everything was so thorough till the end. Now I'm at step 33 with my phone 100% disassembled and not sure if the film that holds the silver disc should stay or go!!