Definition - What does Supplemental Register mean?
The Supplemental Register is a secondary means of registering for a trademark. The Supplemental Register is typically only for intellectual property that does not qualify for the Principal Register.
In the context of the law, trademarks registered on the Supplemental Register are not protected by trademark law (like intellectual property registered through the Principal Register is).
Justipedia explains Supplemental Register
Even though the Supplemental Register does not offer protection under trademark law, it can still be useful for companies to use it. This is because having a trademark listed on the Supplemental Register can prevent another company or individual from registering another similar one. So, even though a Supplemental Register mark does not allow an entity to sue another entity for intellectual property theft, it can actually help to prevent copycat companies or products.