This command automates the process of organizing UV islands. UV islands are continuous areas of polygons in UV space that are disconnected from other UV data. This commonly occurs in Atlas-mapped UV sets as well as with UV unwrapped data that was accomplished by isolating areas of the mesh for focused unwraps.
The Orient Pieces dialog has a single list for specifying the Direction to orient each island: Auto, Horizontal, and Perpendicular. Modo specifies Auto, by default, to automatically rotate the pieces in either direction according to which direction creates the algorithically-optimal layout. Selecting Horizontal forces the islands into a more horizontal orientation based on the lengths of the islands. Selecting Perpendicular forces the islands in an orientation that is perpendicular to the Horizontal method.