So there is SM-L; Enter Customer Class.
 
However, the web help doesn't show it in the menu list - goes from SM-K to SM-N, although the Index shows an SM-L which is Enter QC codes...
 
Anyways, you can enter codes here, but they are not in the Lookup in AR-A. I don't see any flags in SD-Q that would be applicable. I would assume a No, Yes, Require type option.
 
Is this a yet-unimplemented feature, or am I missing something?

Interesting.  
 
Just tried a lookup in AR-A again, and it shows them...
 
But it still seems like there should be an option to Require them if you want. Otherwise someone can put in whatever they want. Our new processing/printing/emailing of Packing Slips, Invoices and Statements requires that the proper Class Code is entered.