My Co-Writer Royalty-Free License
A non-exclusive, royalty free license is granted for use in the writing, recording, reproduction and performance of new musical compositions with instrumental and/or lyric melodies created by the purchaser. Any use, file sharing, distribution, re-sale or reproduction of the unaltered tracks, or any part thereof, for any other purpose, is expressly prohibited.
In essence, when you add your copyrightable melody and/or lyrics to the backing tracks they become yours to use as you please.