If (say on a DONEx) D class is one of the fare buckets listed for a flight but there's no availability (i.e. D0) then you need to find another flight with D availability (Dn where n>0).

If the flight does not offer the class of travel, then it books into the lower class, except for AA flights when on a DONEx and business class is not offered, A is used. If no A (such as AeAgle) I believe it books into Y.

There's a comprehensive table in the rules; it would be useful - I'll try to dig it up.