A more thorough explanation:

A substitutional legacy is a type of legacy that is given to a person in a will, but if that person cannot receive the gift for some reason, it goes to someone else instead. This type of legacy is also known as a "substitute gift."

For example, let's say that a person leaves a car to their son in their will, but if the son cannot receive the car (maybe he already has a car or has passed away), then the car will go to the daughter instead. In this case, the daughter is the substitute recipient of the car.

Substitutional legacies are often used to ensure that a gift is not wasted or lost if the original recipient cannot accept it. They can also be used to provide for multiple people in a fair and equitable way.