Use the Extract Pages functionality to extract certain pieces (or parts) from a Song Book or a Score collection for instance.

Access it by opening the "..." menu (on the top bar in Score modeand tapping on “Extract Pages”:

Select the pages you wish to extract and then tap on the "Extract pages to..." button (on the bottom bar).

At this point you have to options :

  1. Create a new part within the current piece (will be accessible from the Parts menu of the current piece). 
  2. Create a new piece (will be added to you music library and accessible as any other piece).