Hi edfmm
I guess they mean
sample gas : 1) a quantity of gas taken from a specific point of the pipeline, used for various calculations e.g gross heating capacity, density, phasmatographic analysis. 2) a quantity of gas taken from a finite space (room, chamber, vessel) used for calculating the existance and the percentage of (natural) gas in this finite space
zero gas : We say that an environment has zero gas when we measure with an appropriate device (explosive-meter) 0% gas.
span gas : Usually the devices that detect and measure gas do not range from 0% to 100%, but let's say from 0% to 4.5% because for natural gas 4.5% is the lower explosive limit. In order to calibrate this device you need to have a prototype gas with exactly 4.5%. This prototype gas is called "span" gas because it is used to calibrate the span of the metering device.
fuel gas : gas which is used as a fuel (sorry, can not make it more simple)