Answer:
When the metal can exist in more than one oxidation state
Step-by-step explanation:
When metal exhibit more than one oxidation sate, we have to write the oxidation number of the metal in front of it to for a better clarity. Most of the transition metals exhibit more than one oxidation state due to presence of d electrons. For example the oxidation state of iron are "+2 and +3"
The oxidation state of copper are "+1 and +2"
The oxidation state of Mn are :"+2", +4, +7. so we generally write there oxidation state in roman For Mn compound in +2 oxidation state we write Mn(II) For Mn compound in +7 oxidation state we write Mn(VII)