What does that mean for the Siegels’ rights to use the character?
It means that if they were to license Superman to another company, like Marvel, that company would not be allowed to call the character Superman on the cover of the comic, nor would they be able to use the changes to the character post-Action Comics #1, like the more detailed S on Superman’s chest or the expanded powers. It’s a lot like how DC has a character called Captain Marvel, but they have to call his book Shazam! because Marvel has a trademark on the name Captain Marvel.
