Christian Standard Bible cross reference system

Tom Klose

For a while now I've been using the CSB in reading and study. What I have found are a unique set of cross references that are unusual or confusing. I see sometimes double letter cross reference system  like aa, ab, ac, Etc and other times I see a, b, c, etc.  What's the difference?   It is confusing. 

JT (alabama24)

This isn't a unique system... it is common. "aa" comes after "z"