From the Rules and Guidelines for Notes and Glossary Entries:
Rule II: Use the Note field only for three types of information: card attributes, identity attributes, and subset information printed on the card.
Note that it says "printed on the card" and not "printed only on the front of the card". That opens that up to indicate that some subsets were only labeled as such on the reverse.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).