Duff is the partially decomposed organic matter between the mineral soil and fresh litter, it is often referred to the F (fermentation layer) or H (humus) layer of soil.
Duff often burns in the less efficient smoldering phase of a fire, it can create up to 50% of smoke from a fire.
Source: Hardy, Colin C., Ottmar, Roger D., Peterson, Janice L., Core, John E., and Seamon, Paula. 2001. Smoke Management Guide for Prescribed and Wildland Fire 2001 Edition. Produced by the National Wildfire coordinating Group Fire Use Working Team.