Here is my answer.
replacement and substitute, relating to objects or products.
Let's say you own a Honda motorcycle, and the wheel falls off. That's not good!
You go to the motorcycle shop to buy a new wheel.
If they have the exact same Japanese Honda wheel, identical to the original, that is a replacement wheel.
But if they have a Chinese wheel that fits your bike, looks almost identical, and saves you money, hey, you buy the substitute wheel.