A #"formula unit"# is in effect the #"empirical formula"#, and it is particularly used to describe non-molecular species.
Explanation:
And the #"empirical formula"# is the simplest whole number ratio that describes consitiuent atoms in a species.
Typically, we use #"formula unit"# when we describe the composition of non-molecular entities such as salts. Common salt, sodium chloride, is certainly not molecular. Its description by its formula unit as #NaCl# is unambiguous.