User blog comment:Xilinoc/Ch. 553 - Frozen Cross/@comment-10330822-20131016154408/@comment-1248963-20131017053404

Pretty certain they established that the letters do indicate some level of rank (when Ishida was appointed "A" -the same letter as Yhwach- even though everyone expected Haschwalth "B" to be the successor). It may not be strictly power-based, but it definitely indicates a sort of hierarchy.