If something is substitutive, it can take the place of something else.
Substitutive things are like backups or replacements.
Examples:
This toy is substitutive for the one you lost.
I need a substitutive for my broken pencil.
The substitutive teacher is here because the regular teacher is sick.
I have a substitutive snack for when I'm hungry.
My mom is my substitutive when my dad is at work.
The substitutive bus is here because the regular bus is broken.