Holo fuel is like Eth gas. Holo fuel and HOT are designed differently and have different purposes. Holo fuel DOES operate in a similar way as Eth gas, but there are some design differences. Holo fuel is designed to perform billions of daily microtransactions for hosting services. Holo fuel is Holo's native, asset-backed, mutual-credit currency.
Ethereum, Bitcoin, HoloToken (HOT, ERC20), and Holo fuel are cryptocurrencies.
p8: In the future the micro-transaction flows for hosting payments could also undergird a general purpose cryptocurrency payment system in a similar way that “gas” powers the Ethereum transaction engine.
p37: Holo fuel is a unit of account in Holo using value-stable double-entry accounting. The crypto-credit used to purchase hosting in the shared Holo Ecosystem.
When it is released, you will be able to buy Holo fuel through the Reserve Account, third party exchanges, peer-to-peer, etc.