Question about Approach Plates

One of my students asked me a question the other day that I have been unable
to answer.

Many instrument approaches are labeled with a letter following the primary
name and the letter indicates that the final approach course is not lined up
(beyond 30 degrees) with any runway, and that a circling approach is
required. For example: VOR-A.

That said, what is the difference between VOR-A, VOR-B, VOR-C, etc? Does
the letter have any additional significance? Why, at some airports is there
a VOR-B or VOR-C but no VOR-A? Is it because there used to be a VOR-A and
it was decommissioned?

Thanks,

Russ MacDonald, CFII