No one knows what they mean for sure, but since most of the Britannian Imperial Family members are half-siblings, the suffixes are to distinguish who is related to whom. For example, Marianne is the mother of Nunnally and Lelouch, so each has the "vi" suffix, while Euphemia and Cornelia, younger and older sisters, have the "li" suffix. Last known "pure-blooded" relatives are Clovis and Gabrielle, son and mother, who have the "la" suffix. Going by that, it seems that Odysseus eu Britannia, Karine le Britannia, Guinevere su Britannia and Schneizel el Britannia, three of which are the eldest siblings, have no known half-relatives.