A sidechain is a blockchain with a one-way trust-minimized bridge from the base chain.
The Fuel technology can also run as a sidechain to an existing layer-1. This means there is a message passing bridge between the layer-1 and Fuel. In this configuration, data availability would be handled by the side chain, while settlement is handled by the layer-1.
There would also be an option to run it in a semi-provable configuration, whereby fraud proofs can be used to ensure better validity guarantees using the layer-1 as an arbitrator.