Answer
Zero carbon fuel: Fuel which when used onboard for power generation or propulsion will not produce any carbon emissions in the exhaust and right from synthesis, transportation, storage ie LCA of the fuel no carbon emissions were released.
Eg: Ammonia and Hydrogen
Net zero fuels: Fuels like Biomethanol which are produced from bio ma ss they are carbon neutral as the CO2 is required for producing that bio ma ss is taken from the atmosphere and same is released back into the atmosphere after use onboard after combustion. So net CO2 addition is zero to the atmosphere.