Subsistence agriculture is agriculture practised on a small scale. In this type of farming, farmers mostly consume the entire product they produce. Production is just sufficient for the farmer to subsist at a minimal standard of living. Such farming is carried out on the same plot of land that the farmer owns every year. Subsistence farming is settled agriculture.
On the other hand, shifting agriculture is a slash and burn method of cultivation where the farmer uses a plot of land for a certain period of time after destroying the natural vegetation of that land. Once the natural fertility of that land has been exhausted, the farmer shifts to another plot of land. Like subsistence agriculture shifting agriculture is also small scale. The main difference is that shifting agriculture is not settled agriculture as the plot of land keeps changing.