SKALE is a multi-chain network full of many EVM compatible blockchains. SKALE is know for it's zero gas fees, instant finality, and unique validation and economic models.
SKALE Fuel -- or sFUEL for short -- is the native gas token of each and every SKALE Chain. sFUEL has no monetary value and cannot be purchased on a CEX, DEX, or on-ramp.
SKL is the native utility token of the SKALE Network. It used for delegation, staking, chain payments, and governance. It is not used to pay for gas fees on a SKALE Chain.