Unit formulae (not formula units) are the simplest ratios which we use to represent giant structures, whereas chemical (molecular) formulae are the actual number and types of atoms in a given molecule.
For example, the chemical formula of water is
Sodium chloride or common salt however, is a giant structure and therefore each 'molecule' is an individual salt crystal, with billions or trillions of sodium and chloride ions held together by what we call ionic bonds. Each crystal therefore strictly has a different actual chemical formula depending on how large it is. However, the