Wood flooring is graded at the factory into one of three grades, Prime (also refered to as Select), Natural (also refered to as Character) and Rustic.
Prime or Select grade is the highest quality of the grades and will be more expensive because only around 15% of the timber arriving at the manufacturers will pass as prime grade. It will have very few knots, little or no variation in colour and the grain should be fine, even and relatively straight. Overall, a Prime/ Select grade floor will have a very even and consistent appearance.
Natural or Character grade falls somewhere between Prime/ Select grade and Rustic grade. There will be a distribution of small knots, burrs and other minor imperfections plus some colour variation giving the floor a distinctive character.
Rustic grade allows for through knots, small cracks and a reasonable amount of colour variation. A Rustic grade floor has bags of character and also represents excellent value for money because this is what the majority of timber will be graded as.