A more thorough explanation:

A blood-grouping test is a type of test used to determine the blood group of an individual. It is also used in paternity and illegitimacy cases to determine whether a particular man could be the father of a child.

For example, a genetic-marker test and a human-leukocyte antigen test are two types of blood-grouping tests used in paternity cases. These tests help to eliminate men who could not be the father of a child.

For instance, if a man's blood group is incompatible with the child's blood group, then he cannot be the father of the child. Therefore, the blood-grouping test helps to establish the likelihood of paternity.