There are two types of Licenses - Exclusive and Non-Exclusive.
An exclusive license provides the purchaser the exclusive right to use the vocal track in a new song. This means that the vocal track can no longer be sold to anyone else. Voclio automatically takes down the vocal track when and Exclusive license is sold. Purchasing an exclusive license only ensures that the vocal track may no longer be sold to anyone else thereafter - it doesn't mean that a non-exclusive license hasn't already been sold. Be sure to message the vocalist if you want to ensure the license hasn't already been purchased.
A Non-Exclusive license allows a vocalist to sell the same vocal track to multiple parties. Typically non-exclusive licenses are much cheaper than exclusive licenses. This allows producers to purchase vocal tracks at a lower cost while allowing the vocalist to earn recurring income on the continued sale of a vocal track.