Enter 'M' or default to define the centre of mass of the molecule or 'C' to define the centre of charge. Dummy atoms can be removed in Book 5 . These Centres can be used for generic measurements, overlay points and planing centres . They are often quoted in the literature as measuring points for spectroscopic or other experimental data. The centroid appears as a brown point linked to the nearest heavy atom.
Normals are useful to align complex groups and cycles.
After picking the 'Normal to Plane' button, pick 3 atoms in the chosen plane of the normal. Then pick the atom onto which the normal will be constructed. It is often useful to construct a normal at the centre of a ring. A tip is to create a centroid for the ring and use this from which to project the normal.