Originally Posted by
MSPeconomist
IIRC there was a discussion about what fare class availability is required to do SDC in C+ given that W fares seem to have an underlying coach fare basis. OP should check DL's current published rules even if just to have an argument when an agent refuses and says to just try SDS.
C+ fares can SDC onto any flight with open C+ seats. The issue seems to be if there's no C+ seats. Then some agents insist you need the same coach fare class that underlies the C+ fare.