Board Thread:Questions and Answers/@comment-5836726-20131016213309/@comment-108.54.195.186-20131021135233

They're simply random one-syllable denotations indicating which of the current Emperor's consorts is their mother. Notice Lelouch & Nunnally both have 'vi' and Cornelia & Euphemia both have 'li'. They're assigned them at birth based on who the mother was and don't have meaning beyond that.