What is the difference between mass selection and individual?

There are two types of artificial selection: mass and individual. During mass selection, a group of individuals with the desired traits is isolated. The offspring with this selection is genetically heterogeneous and therefore gives a splitting of characters during reproduction. In this regard, the selection is carried out in a number of generations.
During individual selection, isolated individuals with valuable qualities are isolated and their offspring are separately grown. With subsequent self-pollination in plants or closely related crosses in animals, clean lines are drawn. The clean line is a group of genetically homogeneous (homozygous) organisms that represent valuable source material for selection