That is a good question.
I wouldn't know anything about an official interpretation of the terms, but I myself do intuitively distinguish between the two. I have lost some Salvia species when i planted them in part shade, while another patch in dappled shade thrived. The way I see it is that it has to do with the plants capacity to pump up water in dry air conditions. The salvia's that were in three hours of continuous sun, either morning sun or midday sun just got wilted leaves the moment the sun hit, and that progressed to dead leaf edges and finally the plants dying. The ones in dappled shade also had wilted leaves sometimes, when patches of sunlight hit, but because sun and shade alternate throughout the day, the evaporation never truly outpaced the water their roots could pump up. They did better than the ones in full shade, which also survived, but never really grew much.