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Hello

I am confused on the letters assigned to an IAP. I understand z and y. I don't understand the difference between the following at Scottsdale airport:

RNAV (GPS)-D
RNAV (GPS)-E
VOR-C
VOR/DME-A

I am guessing that if there is an rnav circling IAP then it will have an "E" or a "D". I cant figure why. A non rnav circling IAP will be a " A" or a "C" and I don't know why.

Thanks