In chemistry, if you talk about Iron (III) chloride you are talking about FeCl3. In this molecule the iron (Fe III ) is in the 3+ state, that is, it has lost 3 electrons and the clear notation for this is Fe3+.
In astronomy, often the Roman Numeral notation, for example Fe III is used to mean the Fe2+ state where two electrons have been lost.
Where a chemist would use Fe or Fe0 to describe neutral iron which has not lost an electron, an astronomer might use FeI ( usually when describing spectroscopic data), to mean the same thing.
So, if you see Roman Numerals to describe atoms / ions, consider the background of the author and the subject that you are reading about to determine which state is being discussed!