What is subsistence agriculture?
If someone harvests products for his/her own demand, it is called subsistence farming.
Think about the question "How do we exist?". (1) survival and (2) population growth.
How do you survive? Consume food, drink water, inhale, take shower, etc., etc.
If you want to survive, even if you do not have money, you must find food and shelter. It is better if you grow your own food.
It is called subsistence agriculture. If your products are good for yourself or your family, you can survive (even if you do not have any money).
According to Open Geography (2017), the definition is:
"Subsistence agriculture is the production of food primarily for consumption by the farmer and mostly found in less developed countries. In subsistence agriculture, small-scale farming is primarily grown for consumption by the farmer and their family. Sometimes if there is a surplus of food it might be sold, but that is not common. In commercial agriculture, the primary objective is to make a profit."
Open Geography Education (2017). Retrieved on the internet: www(dot)opengeography(dot)org(slash)ch(slash)6(slash)food(slash)water(slash)and(slash)agriculture(dot)html