Soil classification is a unified system of soil. Each soil can have a place in the system and there's only one place for each soil.
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Soil individual is a three-dimensional entity in natural landscape, characterized by certain position, size, slope, look, horizons and basic properties. It is composed of a group of single pedons with statistical similarity in a certain area.
Soil taxonomy is based on soil forming factors, soil formation processes and soil properties.
The current trend of international soil classification is quantification, standardization and internationalization.
Diagnostic surface horizons locate on the uppermost layer of soil profile. The diagnostic surface horizon does not necessarily correspond exactly to the genetic layer A. It refers to the general surface layer including both the narrow sense of A layer and the AB layer (in transition from layer A to layer B) in soil formation.