I think it means that your level doesn't matter. Even if you're level 150 you can still use it, cause they do have level restrictions on quite a few things.
It also probably means that as long as you have the crystals, you can keep using them. Not like if you use a crystal using another one is useless which a few of the other unique items seem to be like the fire crystal thing. Which is why he was giving the extras to the assassin girl and his sister.