Prestige Class differences in Fare Basis Codes
 

I'm a Dl Plat considering a switch to KE, my standard run is HNL / BKK.
I usually buy the M on DL, 50k miles, and it runs $3500 nowadays.
I just booked for 12/1-12/8 and they had to sell me I/S on my return, no M class fares available to get upgrades with. So, the ticket is $4.3k. I knew KE had a cheaper product in pure Biz, and I've been told the actual product is better quality, so here goes.
I booked into Prestige and I got two choices, Prestige Promotional $3.855k with this Fare Basis (IXBZ) and Prestige Special at $4.123k with (DXBZA)

Does this just translate to an "I" and a "D" for purposes of Delta mileage multiplier and MQM's?

Yes. Generally the first letter of the fare basis gives you the booking code to look up in earning charts. The exception would be some of DL's UP fares that book into A or P but have fare bases beginning with H or Q. Those are supposed to earn as A and P, iirc.