What is C+ fare code? I've seen it listed as C, I, W and S
Pre-accelerator the MQM for C or I is 150% of miles flown.
I'm pricing an itn and C+ is listed as S--would I earn 150% or per various fare code charts only 100%?
Per the accelerator promo, C+ will earn in addition a 75% bonus miles toward MQM (just like all other premium cabin fares), this promo only references the NAME of class not a fare code.
What's the "story"?

C+ gets no bonus. Fare classes can have different meaning for domestic and international and for partners flights. Basically unless you're flying international and in Premium Economy (not C+) dont expect a bonus outside of First/Business class.

Delta Comfort Plus will have a primary rbd of W or S (if paid fare). It will earn 100% of the distance (base), plus 75% (2021 bonus), for a total of 175%.

J/C/D/I/Z are used for First Class/Delta One. Initially, Comfort+ only booked into W class. S booking class was added more recently. DL Premium Select books into P/A/G fare classes. Basic Economy and Comfort+ have Fare Basis codes which start with coach fare classes as the fare filings basically mirror the main cabin fares (although Basic Economy fares only mirror main cabin fares up to a point). There's a dual inventory check when booking Basic Economy and Comfort+ fares and there must inventory in both their booking class and the underlying coach fare bucket (so when they zero out one of the coach buckets, it automatically moves you up to higher Basic Economy and Comfort+ fares along with a higher main cabin fare). Domestic FC fares also use coach fare classes for fare basis codes and have dual inventory checks. Earnings are based on the booking class (not the fare basis class).

Although under the temporary status accelerator promotion, Basic Economy and Main Cabin get a 50% bonus and Comfort+, Premium Select, First Class, and Delta One get a 75% bonus. The bonus is applied after the cabin bonus, so the MQM chart ends up looking like:

Basic Economy: 150% of miles flown
Main Cabin (most fare classes): 150% of miles flown
Main Cabin (Y or B class): 225% of miles flown
Comfort+: 175% of miles flown
Premium Select: 262.5% of miles flown
First Class or Delta One (most fare classes): 262.5% of miles flown
First Class or Delta One (J or F class): 350% of miles flown