Different Fuel configurations and how to use them

I see there are tons of configurations of how to use Fuel, as described here. Does it mean that there will be multiple instances of Fuel blockchain (one per configuration)?
Or there will be just one instance and it’s up to an application to decide on how to use it? If so, then how exactly an app can specify the configuration it will use? Should an app developer deploy any smart contracts on Ethereum if he decides to use Ethereum as a settlement layer?

I’m probably lacking some understanding of the concept as a whole. It would be helpful if anyone could clarify it in details :slight_smile: