First, to be clear, when you purchase ANY rights to a track, non exclusive or even exclusive, YOU ARE NOT BUYING the composition/ instrumental/copyright. You are buying the rights TO USE THE WORK. 
 

When you purchase a non-exclusive track, you are essentially leasing or renting a track to use it for your own personal use, but the track may also be leased to other clients, at any time.  Once you record your vocals to a track that was licensed to you, that new song is called a "derivative work." Even though you just made a new song/recording, you only own the copyright to the lyrics and any other elements you ADDED to the original. All royalties and splits should reflect these facts.
 

You can use a non exclusive track for things likes demos to shop record labels or publishers, promotion, mixtapes, public shows or performances, videos, contests, radio play, background music for your podcasts and social media platforms , instructional videos, tutorials, and even can have access to commercial distribution opportunities...depending on the agreement you choose for your particular situation. We just ask for a written/link with producer/site credit. i.e. "Produced By 3rd Sun."
When you purchase the “exclusive rights”, the track is removed from all sites and no longer accessible to any other entity. It is officially "off the market." You will have sole access to the track and all its original elements (stems)...and will have the right to edit, manipulate, or remix it as you see fit  Again, this exclusivity does NOT mean you own any of 3rd Sun's rights to the music, including publishing or the writer's share, unless officially negotiated so.