Not a proper OT feature, is controlled by other features. In the case of roman, you can type any regular number, e.g., “2013” and by using the “ss01” (stylistic set 1) feature in an OpenType aware program such as InDesign, it will automatically be converted to a proper roman numeral. In the case of 2013, the ss01 feature would give you MMXIII without you having to know how to type roman numerals or convert arabic numerals to roman numerals. All of the fonts that have the smcp feature also have the ss01 feature to create roman numerals. Because it’s controlled by OpenType features, if you switch to a font that doesn’t have roman numerals, you will still see the original numerals typed.