Flip Faces
Separately

Flips the face normals of the selected faces to the opposite direction.

Regions

Flips the opposing face normals in each selection region so that all faces normals in that region will point to the same side. The majority of faces in the region defines to which side the normals will point. If none of the faces is selected then the whole object is considered as selection regions defined by one or more groups of adjacent faces.