Edit > Select > By Angle selects all polygons whose normals diverge from the normal of a given plane by n degrees or less. See Methods of Superimposing Planes.
The controls are:
Separation field (0 to 180 degrees) - specifies the value of n. When Separation is 0, the selected polygons are those whose normals are identical to the normal of the given plane. When Separation is 90, the selected polygons are those whose normals are 90 degrees away from the normal of the given plane.
Align Plane group - controls the superimposition of a plane on the polygon object by any one of five methods. See Methods of Superimposing Planes. After the plane is superimposed, an arrow appears in the Viewing Area to indicate the normal direction of that plane. The Separation value is measured against this arrow.
Align button - superimposes the plane according to the method specified in the Align Plane group.
Flip checkbox - reverses the direction of the plane’s normal, usually causing a different set of polygons to become selected.
OK button - terminates the dialog and leaves selected polygons selected for further processing.
Cancel button - returns the object to its previous condition and terminates the dialog.