Each music sheet that is published at MuseScore has a label that shows its copyright protection defined by the creator/uploader of the music. Most songs that can be found and downloaded in MuseScore Sheet music are labeled with "All Rights Reserved" Also all these songs can be found via "Discover"in the Songbook. (I guess).
In total it seems everybody has access to 13670x10 files (till today: the data base grows).
If the added label is wrong then the copyright holder can warn MusicScore to block the content (via: Report Copyright Infringement).
Most songs shows "All Rights Reserved" which has no jurisdiction. It's just a formal sentence sayïng it's not free to use, but even when nothing were mentioned it is already not free to use unless you have the explicit admission of the copyright holder. If you want your uploaded work specificly to be free of copyright you should add "CC0 1.0 Universal" (CC Zero). If you think the work is already in the public domain, like e.g. with most old classical works, then you can add "Public Domain Mark 1.0". Practically I see most people don't care they just use the default mentioned Creative Common Licence "None (All Rights Reserved)" even when it is copyrighted.
I just got my copyright infringement report for "Rose Of Picardi" and the file is blocked. The song was composed in 1916. In the USA, European, the Canadian and Australië laws everything stays under copyright untill 70 years after the death of its creator. However according to the USA copyright law the exception is that everything created before 1923 is in the public domain. The composer Haydn Wood died in 1959 which is 56 years ago but he composed it before 1923. The British company "Faber Music o/b/o Warner/Chappell" reported infringement. Absolutely absurd of course, like if the Internet stops at the boarders. Two different kind of copyright's laws is useless. There are still countries that still use the previous 50 years law. Yes, in 2011 the copyright law was extended to 70 years in Europe and now in 2015 also in Canada. All done by the lobby tryïng to convince politicians it is fair to get money for doïng nothing. It is almost never the creator that earns the money but the middle men.
Happily enough we can protect ourself by using it only for private use and then share it of course with ....eh, I won't tell you.