What is multiple optimal solution in assignment problem?

An assignment problem can have more than one possible combination of
assignments, it is known as multiple optimal solution. But all possible combination
have same final (optimal) answer.
After row minima and columina steps are performed and minimum number of line
covering all zeros and size of matrix are equal assignment can start. In process of
assignment if every row and every column has minimum two or more zeros then
multiple optimal solution exist.