Flip Normals icon Flip Face Normals

The Flip Face Normals action flips the normals only on the selected face(s).

Inside of cube is visible after Normals change

This differs from the Flip Normals action, which flips the normals on every single face on the Mesh.

Tip: You can also use this action with the Alt/Opt+N shortcut, or from the ProBuilder menu (Tools > ProBuilder > Geometry > Flip Face Normals).