If something is substitutional, it means it can take the place of something else.
Substitutional things are like substitutes in a game. They can be good or bad, depending on the situation.
Examples:
The substitutional player went in for the injured one.
I need a substitutional for my broken toy.
The substitutional food was just as good as the real thing.
I don't like the substitutional teacher, I want my real one!
The substitutional robot could do all the work for me.
The substitutional cake was made of cardboard, but it looked real!