Parchment was originally animal skin (other than cow, that is vellum) that had been stretched, scraped and then dried under tension and scraped and pumices some more. These are used to make books, manuscripts, scrolls and various works of calligraphy and/or illumination and can last over 1,000 years. Today however the beginning scribe faces a world where many things are called parchment that aren't at all the animal skin kind and it can cause a lot of confusion.
As mentioned above, there are a plethora of things called parchment that are not suitable for doing SCA scribal work on.
The stuff in the baking/cooking aisle isn't the kind of parchment we are talking about. It is baking parchment. Great for various baking purposes most likely but terrible for SCA Calligraphy and Illumination projects.
The stuff you get at the fine paper section of the office supply store, is great for printing out your resume but it isn't the kind of parchment we are looking for. It is printer paper called parchment paper that is of heavier (and sometimes lighter,) weight, and superior quality that has some color added to it to make it pretty. It is perfect for use with your printer to put your resume on it. It is terrible for SCA Calligrpahy and Illumination projects.
In short if it isn't an animal skin then it isn't the kind of parchment that is being talked about being used by the scribes in the time period the SCA covers.