A cover song is a new recording of a composition that you did not write or of which you do not have legal ownership, such as an a cappella recording of "Poker Face" by Lady Gaga.
To distribute a cover song in the United States, you need to procure a mechanical license.
If the composition that you are trying to cover is in the public domain, you do not need to request a mechanical license. Stem supports obtaining mechanical licenses for covers in our upload flow thanks to our integration with Loudr. You can learn more about how to release a cover song here.