VERIFIED SOLUTION i

MailStream Plus Standard Mail non-auto Flats mailing. USPS clerk says that you have to have 125 pieces to a destination to get ADC rates.

MailStream Plus™, all versions, all platforms
The USPS Qual report showed that 12 pieces were getting the ADC rate.   The USPS Clerk said that you had to have 125 pieces minimum to get the ADC rate, otherwise they should get the MADC rate.

The applicable DMM references are (DMM references as-of May 2019):

DMM 245 (preparation) 8.3e says 10 pieces can make an ADC bundle.
DMM 245 8.7c says that all it takes is one bundle to make a 3dg origin sack. 

DMM 243 (rates) 5.6.3b says that non-auto flat pieces in a 3dg origin sack get ADC rate.  (In this mailing, the pieces were in a 3DG sack to 458, which per L005 is an origin sack for the entry point 430. 

So the pieces were eligible for the ADC rate.
UPDATED:  May 16, 2019