Please explain when and where must we use RHS congruency criteria
RHS stands for Right-Hypotenuse-Side.
Two right-angled triangles are said to be congruent by RHS congruence rule if the hypotenuse and one side of one right-angled triangle is equal to the hypotenuse and one side of the other right-angled triangle.
Thus, RHS rule is used when both the triangles given are right angled triangles in which one side and hypotenuse of one triangle are equal to the one side and hypotenuse of the other triangle.