This is exactly right. Letters at the back of the alphabet moving forward are multiple approaches of the same type to the same runway. For example, if there are two ILS 13C approaches (two approaches based on the ILS to runway 13 center) they will be titled ILS Z 13C and ILS Y 13C. If they build a third one, that becomes ILS X 13C, and so on.
Letters from the FRONT of the alphabet are used for Circling only approaches. "A" is used for the first circling only approach, "B" for the second, and so on. Unlike the X,Y,Z example above, the A, B, C (circling approaches) won't have a a runway in the title. (VOR A, TACAN B, VOR/DME C versus ILS Z RWY 13C). Also, unlike the X,Y,Z's, the A,B,C's can show up on different types of approaches at the same field.
In other words, a field could have a VOR A, and a TACAN B, (there wouldn't be a VOR A and a TACAN A).