It can be both, but it's mostly a blended additive. It is a fuel that is a fuel. Ethanol, for example, is a fuel that can run at the very high level of E85. A lot of vehicles in Canada can run on that level.
It's also an octane enhancer. It would accomplish the same goal as an aromatic or a benzene or what we used to use lead for. Ethanol is a very inexpensive and a very clean octane enhancer.