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As Peter describes, both work within the class much as they do for an interface, especially WRED.
Some differences:
Within class-default you can use both, recall for interfaces it might be either/or.
Within CBWFQ, its class, whether single FIFO queue or multiple FQ queues competes for bandwidth with other explicit CBWFQ classes. How this bandwidth is proportioned varies per IOS version. For example, HQF FQ is is just FQ, prior to HQF, on many IOS based routers, FQ is actually WFQ (like an interface FQ).
