What you are checking for
A certificate of quality lists the tested parameters, the method used for each, and the result. Reading one means checking three things: that every parameter the specification requires is present, that each result is within limits, and that the methods cited are the ones the standard names.
Gasoil and EN 590 diesel
Sulphur content, cetane number, density at 15°C, flash point, CFPP or pour point, water content, and appearance. Sulphur is the one that most often disqualifies a cargo — a 10 ppm specification and a 50 ppm result are different products commercially.
Jet fuel and ASTM D1655
Freeze point, flash point, density, total acidity, smoke point, and electrical conductivity. Conductivity is easily overlooked and is a safety parameter, not a commercial one.
Fuel oil and ISO 8217
Viscosity at 50°C, density, sulphur, water and sediment, micro carbon residue, aluminium plus silicon, and pour point. Aluminium plus silicon indicates catalytic fines, which damage engines — a bunker buyer checks it first.
Crude oil
API gravity, sulphur, BS&W, pour point, and total acid number. TAN matters for refinery corrosion and is often the parameter a refinery buyer cares about most.