What does a roman numeral tell you when it is written after an element

The correct answer is: Oxidation number

Explanation:

According to the IUPAC nomenclature for the inorganic compounds, roman numerals are used to denote the oxidation number of a positively charged ion, called cation.

This rule is only applicable to elements that can form more than one cation with different oxidation state or charge.

The oxidation state or charge of each cation of such an element is denoted or represented by a Roman numeral in the parentheses followed by the name of the element.

Therefore, the oxidation number of a cation is denoted by a roman numeral which is written after the name of the element.