digit vs II

Other hyponyms of digit include binary digit, octal digit, decimal digit, duodecimal digit, hexadecimal digit, significant digit, significant figure, 0, cipher, cypher, nought, zero, 1, ace, I, one, single, unity, 2, deuce, two, 3, deuce-ace, III, leash, tercet, ternary, ternion, terzetto, three, threesome, tierce, trey, triad, trine, trinity, trio, triplet, troika, 4, four, foursome, IV, Little Joe, quadruplet, quartet, quatern, quaternary, quaternion, quaternity, tetrad, 5, cinque, fin, five, fivesome, Little Phoebe, pentad, Phoebe, quint, quintet, quintuplet, V, 6, Captain Hicks, half a dozen, hexad, sestet, sextet, sextuplet, sise, six, sixer, VI, 7, heptad, septenary, septet, seven, sevener, VII, 8, eight, eighter, eighter from Decatur, octad, octet, octonary, ogdoad, VIII, 9, ennead, IX, Nina from Carolina, nine, niner.