Polygons > Primitives > Project Region moves a selected set of polygons until they strike an existing Plane or Cylinder object (as created by Polygons > Primitives > Create Plane or Create Cylinder), and stretches one band of surrounding triangles to form a wall along the projection path.
The controls are:
Object to Project on drop-down menu - displays a list of existing Plane and Cylinder objects into which the selected polygons can be projected.
OK button - completes the operation and closes the dialog.
Cancel button - closes the dialog without projecting the selected polygons.