What is the cross between the progeny of `F_(1)` and the homozygous recessive parent called? How is it useful?

Explanation

When a progerny of `F_(1)` is crossed with the homozygous recessive parent, it is called test cross.
Test cross between pure dominant (A) and hybrid dominant (B) individuals with recessive parent is shown below

Such a cross is useful to determine the genotype of an unknown trait, i.e., whether it is heterozyous or homozygous dominant for the trait.